That's really surprising. It would be interesting if you took your character into one of the really laggy areas and see if you got similar performance for the Mac. Make a villain and run him over to Grandville right under Recluse's statue. I've seen some really high end machines bottom out there. Or better yet, do the Roman Taskforce (forget what it's called atm) and have your 8 person team walk that really laggy road to the big robot.

The CoH development team doesn't seem to do that great in optimizing certain areas within their game world or in large missions where lots of effects are running. I suspect that the Mac client would be just as susceptible to those laggy areas as PCs.