i went for carbine gassed 10.5 with a 1:7 twist rate. with a T2 buffer and quality buffer spring, it runs like a champ. go juice and 0000 grease are your friends.
haven't done any gel tests or anything like that, but i'm extremely happy with performance out of this build. without a CRD, the concussion in enclosed spaces is unpleasant even with earpro, though. i like the SI oppressor, it redirects the blast forward without nulling out your compensator.