For BioShock, the resolution you're running the game at affects performance the most, though on the MBA the maximum resolution you can set is already quite low. After that, the settings that affect performance the most are Shadow Maps and Real Time Reflections. If turning those off doesn't pick up performance to a level you're happy with, have a look at the 'BioShock Performance Tips' section of the ReadMe (it's on the DVD). Hope that helps!
Ian