I've been through this with my BR-AR. The biggest change was reloads. Factory ammo, even Blackhills, is not up to par. The day I loaded 24.8gn of RL15 and added a 69gn HPBT Matchking, I had a new rifle. The trigger upgrade helped, but not nearly as much as the ammo.
My AR is a RRA lower, RRA 2 stage trigger, and White Oak Match upper (free float, Wilson 26" SS barrel etc).