I'm running the 30" ACD and it is fantastic. The $99 adapter pissed me off though. I've never had this much screen real estate in my life - 27+30.
I honestly couldn't tell you if the performance has suffered. I used the computer for a couple weeks without the 30" and now for a couple weeks with it. Can't say I've noticed a difference.
I push my machine pretty hard too, right now running LR4, CS6, Make MKV, Handbrake, Cyberduck, Safari, Mail, Address, iCal, Quickbooks, and Quicktime. Haven't rebooted in a week either... 🙂