Answer is, it depends.

With shammies, being able to basically spam like 2 abilities with a FS/LB thrown in from time to time makes it stupidly easy to do decent DPS in a multiboxing scenario. Throw in classes like druids/mages who have procs to watch for and have to change rotations to maximize DPS and your potential goes down dramatically. Then you have classes with a bunch of abilities on cooldowns that require fairly complex prioritization and do not lend themselves well to condensed macro rotations, and it just gets worse.

Don't get me wrong. The worst of the multiboxer guys that do heroics (like me) are still way better than your average dumb-ass PuG member in blues trying to do heroics. 5 people at 75% capability is still far better than 3-4 guys at 90% and 1-2 guys at OMGRETARD.