A little late here, but it should be mentioned that back when it came out, the Small Net Builder site did testing on MU-MMIO, and found it mostly not working on early hardware, despite the advertizing on the boxes, and it rarely making a practical difference, performance wise. He's kind of having a deja vu effect with the latest wave of routers, but that's another story.
C7's tested well back in the day, in radio performance in real life environment tests over distance. The antenna/radio combo did well, so that makes them decent AP choices. It's still worth putting OpenWrt on them, for the cutting edge AQL code improvements make-wifi-fast on the ath9k radio, and since about the beginning of April, the ath10k (5ghz) radio as well.
Personally, I've had better luck than most, and better than I've had before, running the April 3d snapshot that includes the ath10k AQL. Running 40mhz bandwidth on 5ghz may be helping, as well.