Bard: The 'one mafia per neighbour group' game finished up on Megatokyo, with a scum win. Town completely dropped the ball with two no-lynches and the 'vigilante' group failing to hit at all over three nights, and medics failing to cover confirmed town players, so, I, uh, well I don't know if that's the kind of playtest you want to consider. We did some things a little differently to your stated set up - we were told that there was guaranteed to be no more than one mafia per group, but no knowledge of how many mafia there were total. Also, each group had it's own unique role - one was a detective, one was a medic, one was a vigilante, and one was a backup group.
You can check it out in detail
over here, but if you have any specific questions and don't feel like trawling through the thread, send me a pm.