You would, you wouldn't have anywhere near the balance and momentum needed to ride up obstacles with a 22/52. Riding technical uphills is not a low gear ratio game, it's a 'you need enough power and stamina to not use the granny gear to have enough speed' game. Even a 22/32, based on my experience, is often not a high gear enough and you get hung up on stuff.
As for the weight, Shimano's 11-40 XTR cassette weighs 327 grams. Sram's 10-42 XX1 cassette supposedly weighs 268 g. The 12spd Eagle XX1 cassette weighs 354 g. I don't think there's much of a benefit in unsprung weight by running a 2x system. Not when you take into account the higher overall weight due to an additional front sprocket, derailleur, shifter, etc. and when you see, the difference is ~100 g, if at all (XT cassettes are much heavier). After all running a DH casing tire will be a bigger difference compared to a thinner single ply tire.