Dry fire/dry-drawing from holster do this for about a 30mins a night for a week and you will be amazed how easy you aquire the ability to see the dot instantly. Just repetition and it will come.
Does your new barrel have a rifle extension and your upper m4 ramps? This could be happening during feeding. Drop a few from a locked back bolt extract and check.
Change of powder amount is probably not necessary just check with your chrony but as others have stated depending on your powder choice you may get excessive pressure signs when switching to SPP.
Can't answer any of your questions relating to stress on it, but I have the rescomp and it is nice. My hands are kinda small for the 2011 platform so any help reaching the mag release is nice.