Providing you optimise your code in a reasonable manner and use interrupts to handle the more critical stability functions to make sure they get done in time. I don't see why a PBP programmed 18Fxxxx controller running at 8Mhz or faster shouldn't be able to handle the job with time to spare.

Whilst there is a lot to do you have inertia on your side. From reading some of the other posts, sounds like the trick is in being able to regulate the revs of the propellers with great accuracy. May be what's needed is two Pic's. One slave to maintain precise Prop revs on the 4 motors and a Master to do all the attitude, direction etc data collection and decision making and simply update the required revs to the slave PIC. when necessary. This will allow the Master plenty of time to gather data and maintain stability.

One other thing. The larger the Quad copter the more time you have to make decisions. The larger machines have considerably more mass and are less twitchy and more stable. Of course this is offset by being more expensive to build. and crash.

Ron...