Your PSA lower should work with LMT, LaRue, etc. Things get a little more complicated with AR10's and off the top of my head not sure what lowers an LMT or a LaRue require, so I'll defer to someone else on that one.
As far as other calibers, you have a bunch of choices even in the AR15 platform. .458 Socom, 6.5 Grendel, 300 BO, 6mm ARC to name a few. Really depends on your intended usage. .458 Socom is a great round if your primary focus is stopping power. But recoil is pretty stout, the bullet is going to start dropping like a rock beyond 100 yards and ammo is going to be pricey and tough to find. 300 BO is great out of a short barrel and if you shoot subsonic through a suppressor hearing safe. Ammo is generally pretty available and probably .60-.80 per round. 6mm ARC is the new hotness. Great for long range shooting. Not really sure about availability or price. At the end of the day it really depends on your application/intended use, how much of a premium you're willing to pay for ammo, how much recoil you're willing to tolerate, whether you're ok with shorter barrel life, etc.
