I went shooting Saturday out in the desert a few days after my original post and played around with my M&P 15-22 and a cheap fixed 4x scope. I was able to hit a 12" gong at ~300 yards about 20% of the time running suppressed with CCI HP subsonics. I had about the same hit ratio (if not better) with iron sights.
To compare this to .223 would be a stretch given the inconsistency of .22 ammo and the trajectory I had to use at 300 yards to make hits, but that 12" plate still looked pretty small in a 4x optic. I'm sure I could hit it pretty consistently with a .223 and a better 4x scope at that range but group size would definitely suffer. I don't doubt your abilities Sam but I would have a hard time identifying a target at 600 m with my cheap 4x scope (Nc Star) never mind hitting it, the ACOG does have much better glass though.
So I'm thinking I'll go with the Vortex Viper PST 2.5-10x44. The 2.5 will be alright for close up, although I still like the idea of throwing a red dot on a 45 degree mount.
One thing I definitely learned shooting out it the desert is that I don't really care for mil-dot or BDC style reticles. My Nc-Star reticle was horrible when it came to compensating for range, and the quick range adjustment on the top is a gimmick as I feel it would be on even a high end scope because it really depends on the load you are shooting. Even the Vortex Viper 4-12 w/DBC that I had on my .300WSM and the SS fixed 10x mil-dot that my buddy had on his .308 were annoying to use unless you knew the exact range and were using the same load each time (my buddy had a suppressor on his .308 and we were shooting subsonics and supersonics out of it). That trip definitely upped my desire for Vortex's EBR-1 MRAD reticle, having the fine dashes rather than the dots and numbered increments would really be a plus.
Now I just need to find one locally so I can take a peek through it and make sure the glass is okay. If anyone has seen one please let me know where.
"The truth is, all might be free if they valued freedom, and defended it as they ought." Candidus (Samuel Adams), The Boston Gazette Oct. 14, 1771