The best upgrade to any AR-15 is an aftermarket trigger. My recommendation is a Geissele Hi-Speed National Match - Trigger Set.
Agreed. I sort of want the SSA-E for my mid-range AR. I currently use an older Rock River two-stage and it’s OK for a general carbine, but my mid-range is shooting dime-sized groups and I think I can still do better.
Yep, excellent choice. An improved trigger makes a huge difference in the AR platform.
I've also taken a liking to the Magpul ACS collapsible buttstock for carbines - I like the way the comb is stationary and the stock slides rearward from beneath that surface.