You could always do a nand and then flash MCMCR over the top. I did without any problems. MCR definitely more stable. Had the occasional force close on MCMCR. Have been wondering if its due to flashing over the top of MCR. Today I did a clean flash on my Nexus 7 and plan to do my GNex tonight or tomorrow.
On my Nexus 7 it is definitely more responsive with a clean flash and so far no FC's.
MCMCR is definitely good to go as a daily driver. With all the extra settings though it does make it easier to balls something up yourself. Wondering if thats why I had the problems also as I never had any to start with.
Speed on N7 with clean flash is excellent. Speed on GNex is acceptable but I expect it to improve with a clean flash as per the N7.
Definitely worth a try. If you don't like it you can always go back to MCR.