... even though the gear box is harder to upgrade ...
I don't see what you mean there? I have the Larue Tactical version, which is essentially the same as the colt with different trades and furniture, and the GB is the same as any other TM compatible gun no?
About your mag release, I don't have that issue at all. Did you try different mag brands? I'm using MAG and they're totally fine and feed very well. Try to put some black sharpie on the sides of one of your mag and put it back and forth in the magwell a few time. If you see some wear of the sharpie marks you could always try to sand or file the magwell just a bit in these locations.