Crankarm length is a "sweetspot" kind of thing. I run 160mm cranks with a 50/36, 11-28 Shimano Dura Ace set up (which shifts great). The longer your crankarms, the more torque you will have for climbing (think of the crankarm as a lever), but the sooner you will spin out on the downhills. Conversely, the shorter your crankarms, the less torque for climbing, but you'll be able to spin up to a higher speed down hills. I tried a 155mm set for a month and did notice the difference. It wasn't a big difference, but during that month I did bag some downhill KOMs that I had not been able to grab with the 160's. But I ultimately didn't like the decreased torque for climbing, and I had to spin a bit higher than I like on the flats to hold my usual pace. So I returned to the 160's and have been happy since then, after experimenting. I found my sweetspot. Yes, I would like to spin faster downhill with a 52, but the 50 works so well for me on every other kind of terrain that I plan to stick with it. When Shimano goes 12 speed for its road groups, and I can use a 10-28, I will go with that and keep the 50t.
Shimano and SRAM GPX bottom brackets look the same, but they are not compatible. Shimano uses a straight 24mm spindle while SRAM uses a 24/22mm spindle.