Changing rebound speed when changing shock air pressure.

I'm not a suspension expert but the answer is yes. Your thoughts on more pressure are correct, as pressure goes up the rebound force also increases so more dampening is needed to prevent that bucking feeling. You may need to reduce compression at the same time since you're using the air spring more. One thing I'd recommend for your setup is go by your sag measurement. Depending on the bike's sag recommendations, I'd set enduro sag in the 30% range and XC in 20% range. 

Great question! You're correct that total spring forced damped is probably equal to or less than your enduro setup, so I can see why you assume the damping necessary is equal, but there's more at play here than just damping your rebound stroke at the deepest part of your travel. Interestingly, most of the high speed rebound damping that occurs at bottom out is provided by shims on your main piston, which aren't adjustable with your rebound knob. Your rebound knob is mostly just affecting rebound speed at the top 2/3's of travel. Generally speaking, faster rebound settings are going to track the ground better and provide better grip, and slower rebound speeds are going to be more stable.

Ideally, you'd run the XC setup with not just more rebound, but even slower by feel than the enduro setup. Downhillers (generally) runner faster rebound than cross country riders because speeding up your rebound is an easy and free way to add traction, and slowing down rebound is an easy and free way to prevent unwanted pedal bob.

An active bike is a bobby bike, all things being equal. Downhillers are happy to make that tradeoff, cross country riders much less so. While it's true that you can ride an active bike smoothly to minimize unwanted bobbing, that takes extra energy and cross country riders are all about minimizing their energy demands wherever possible. If I were you, I'd try a slower rebound speed on your 100mm setup and see how it feels. The bike might feel a little zippier when you're pedaling and pumping. This is also why dirt jump riders and slalom racers run slow rebound.

Literally every cross country bike I work on has the rebound set wide open and feels like a pogo stick. It really shouldn't, but its been a curious observation! My feeling is that they are all so far past needing a service that they are getting a large amount of damping from the seal friction.

The 'theories' all depend on a number of things, not all of which you have control over and it all depends on how deep in to the rabbit hole you want to think about it! the TL;DR is go ride and test several different spring & rebound combinations.

In general, there are a couple of basic rules of thumb I use - make one change at a time, so don't assume you automatically need one change after you've made another. Your shock might be overdamped in enduro settings for all we know? 

Typically when a fast or aggressive rider sets up a bike vs the average dude on the same bike, they will use a higher pressure which creates higher rebound speeds. But they are also hitting things much faster so it kind of resolves itself and they don't need to slow it down either. 

Suspension theory quantifies a set up with damping ratios - which is a measure of how much damping you have relative to your weight and spring. However we have different damping rates in compression and rebound, so if you are increasing rebound damping to compensate for a higher spring pressure you should theoretically increase the compression damping the same amount! This is one of many counter intuitive things people miss - they will easily understand that adding spring rate requires more rebound damping but most people will also decrease compression damping at the same time. So the proportion of compression to rebound damping is yet another variable that is considered when looking for a technically "correct" set up. 

The thing that happens when you have mismatched compression and rebound is the damping is allowing the suspension to settle higher or lower than its dynamic sag point, which creates a slightly unsettled feeling. ie a stiff spring with slow rebound (high natural frequency + stronger rebound than compression ratio) will want to settle high in the travel but in motion it ratchets down (or 'packing down" which is a term I'm not a big fan of) so the bike is constantly moving between these 2 points so will feel very harsh due to not having enough sag, but also being pulled down in to the stiffer part of the stroke. Conversely overly fast rebound + soft spring allows the bike to rebound up higher in to the "softer" part of the stroke while it is also trying to sit down in to its travel, creating a "wallowy" feeling. This can also be very harsh if the compression damping ratio is proportionally higher, because the soft spring allows for higher damper speeds which create excessive damping forces. Note I say the damping ratio is proportionally higher than ideal, the damping might be the theoretically right damping rate for you, but if the spring is too soft then the ratio will be too high because softer springs need less damping to control them. Bearing in mind that adding damping makes the suspension settle in a shorter number of cycles, while your spring rate determines how much time it takes to settle, otherwise known as the frequency.

Lastly, have you actually timed how much faster your xc set up is over an xc race? I have a feeling it might be too stiff and you are actually losing energy/speed to vibration, which might "feel" fast but actually isn't. Bikes tend to be pretty efficient these days and the kinematic is optimised around a certain sag range. If you are running things harder than needed you might not gain any efficiency but lose a lot in traction, control and vibration. If you then slow down the rebound to bring the bike back down to its working range then technically the energy is being soaked up by the rebound circuit! Same goes for the wheel/tyres, simply being light doesn't always translate to fast, you really need to test different tyres and different pressures to optimise the rolling speed on the trail.

Great stuff. I guess I spoke too generally to be 100% accurate. Of course there are a million unknowables, but I was speaking in generalities because they're generally helpful.

Every cross country race I've ever done has been on a consistently smooth surface, with the exception of very brief technical bits, so slower rebound speeds with medium to low tire pressure has definitely been an advantage. Letting the tires take care of ultra high frequency vibrations from the dirt and letting the suspension take care of larger impacts and mass shifts has felt very fast. If you're doing a cross country race on rough surfaces (like the BCBR for example), YMMV and you may benefit from faster rebound or other mods like softer suspension, bigger or heavier tires, higher tire pressure, etc.

Interesting stuff, can you expand on why compression damping should be proportional to spring weight (5th paragraph in quote, can't seem to quote just a bit of your post). As you point out, easy to understand why rebound damping should be somewhat proportional to spring force but I had always thought that compression damping was more "by feel" based on characteristics of the rider/bike reacting to speed specific events on the trail.

Don't know much here and trying to learn, as I definitely tend to run my stuff more open on compression vs. rebound (rider weight 205-210lbs geared up, Fox 38 160mm 29er @ 106psi w/ 3 spacers, LSR 5/HSR 4/LSC 11/HSC 7 plus a 2023 SD Ultimate Coil 550lbs spring, LSC -1, HSC -2, Rebound -5 (out of 20) and HBO 0/neutral). I use 90+% of fork travel every ride and rear end feels "good" but still trying to get a bit more supple feel on both ends of the bike as I ride tech/steep stuff, some drops and very little jumping.

Sorry I meant to reply to this, but I'll try expand on that!

When you increase the spring rate and everything else stays the same, you cause lower damper speeds for any given bump versus the softer rate. Which means the damper is producing less force and doing less work, ie dissipating less energy. This means if the bump energy is the same, then what wasn't absorbed by the damper is either stored in the spring or transferred to the rider. So you would end up adding excessive rebound damping to compensate, which would cause problems of its own. 

Feel and preference still plays a part in it, but thats mostly driven by spring rate, where increasing the spring makes for a more "playful" feel because the damping ratios are lower (and natural frequency higher) despite not changing the damper. 

All interesting stuff! Makes total sense and written out like that.