I recently made three changes to my carbine that made a world of difference in handling.
1. Replaced A2 buttstock with Cav Arms A1-length stock. I didn't think 5/8" would make a significant difference, but it's a huge improvement for under $30, it handles sweat better, and it's significantly lighter.
2. Replaced CQB Solutions three-point sling with CQB single-point sling and GG&G adapter plate. I never really liked the three-point sling; it was comfortable if all you were using it for was to hang the weapon, but I felt it was slow to present from hands-free, and there were too many straps and loops and such to get in the way. The single-point sling is IMHO easier to put on, easier to go from hands-free to presentation, easier to maintain muzzle control, and it stays out of the way when shooting. On me, at least, it tends to cover your feet with the muzzle, but better mine than somebody else's, which always seemed to be the case with the three-point sling.
3. Replaced HBAR upper/M4 handguards with Model 1 lightweight upper/standard handguards. This was such a huge improvement it is difficult to believe. The weight difference doesn't look like much on paper, but it feels much, much lighter, and the balance is totally different (in a good way). Maybe I'm a girly-man , but after a mag or two with the HBAR I could feel that front sight sagging. None of that with the lightweight. It's much easier to hold up with one hand (mag changes, malfunction clearance, opening doors, etc.), which is nice. It bounces a little more on the single-point sling, but it's more comfortable to wear. The pencil barrel doesn't seem any less accurate than the HBAR, at least at short range (haven't been outside with it yet), but it does get hot fast. I like the standard handguards better than the M4; I thought I wouldn't, but they just seem to be more comfortable for me.