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We need someone to get one of these and do the tests. https://vo2master.com/ Properly measure the amount of energy burned at set watts to compare different set ups. Then we know what is actually more efficient.

"[S]ame watts..." Here's one problem that the mtb science scene has struggled with (and that some media people who should have known better have spread misinformation around). How do we measure effort? Given some riding challenge, which of two different setups lets the rider complete the challenge with less effort? Or controlling for effort, which of two different setups gets the challenge done faster? Even when we don't care about the actual number from quantifying effort, we need to know it in order to make it the same across the test, and power meters don't really give this information. Heart...

We get it, rolling resistance and geometry make a bigger difference than weight, but the OP was asking about weight, ceteris paribus. All else being equal. This calculator is for a road bike, but could prove to be useful: https://www.broleur.com/hill-climb-calculator/ So a rough estimate of a 2kg, or about 4.5 pound weight savings in your bike requires less than 2% increase in watts if you weigh 86kgs (190 pounds) kitted up and your bike goes from 34 pounds to 29.5 pounds. Thats a 6 watt difference. Seems pretty insignificant, until you realize thats roughly the claimed efficiency decrease of adding...

This. To re-iterate much of what Suns_PSD said, MTB speed is primarily related to tires (rolling efficiency & weight) and wheel weight (rotational mass). After that, pedaling efficiency and weight make up a smaller percentage. The role overall bike weight plays is related to how much climbing and how steep the climbing is. Very long, steep climbs, then weight is obviously going to play a bigger role. However, say your average male rider is 175lbs + 8lbs of gear, water + 34lb bike. That's a system weight of 217lbs. Dropping down to a modern 'downcountry/Xc bike might mean a system...

The speed difference between a WW (Weight Weenie) XC bike and a good pedaling AM bike, is 70%+ in the tires. Maybe 20% in the rotating bits (mostly wheels) and then just a bit in the travel and extra chassis weight. As an experiment some years back, I removed the drivetrain (wheels/ tires/ rotors/ cassette, basically just swapped the wheels over) from my WW Spur to my AM SJEvo and did an all out timed loop on a 35-40 minute section that is pretty XC style. It for sure had rocks and a handful of small drops/ ledges (1-2.5') but...

The parameters for my list were they had to make a legit, full suspension, non-e-bike frame.

The list of shocks that historically used ball joints: cane creek DB, ohlins ttx, push 11/6, ext storia for G1, even though I'm positive that you can ask them to build one for other frames as well.

Likely end of the year or early next year. That's the info I was given.