The upper I have was is a complete fully built upper from factory from AA. I've seen those conversion GP kits too and was thinking of going that route but decided against it for that very reason you mentioned. I've read user's reviews regarding carrier tilt on the GP set up and was leary but than later found out that the issue was addressed with beefier BCG and the one piece design. I'm hoping that design was infact redesigned to fix the tilt issues.
Guess I'll find out sooner than later. I dont plan on using the AR on a monthly basis and it won't be in use as much as my pistols as the drive to the nearest range that allows rifle calibers is roughly 30-45mins away. The AR will mainly be a safe queen so to speak so I'm not too worried about the issues just yet. If the time comes I'll address them then.
Now regarding the piston spring, are those easily replaceable/removable? The AA set up I have the spring looks as if it doesnt come off, maybe i just overlooked it.