We have a "test" 27" iMac that's running a Blackmagic UltraStudio Express with Premiere Pro CS6. It works just fine for playback, but when you start fast-forwarding or rewinding your footage it can't keep up. Where your playhead is in the clip can be 10 or more seconds off from what you are actually seeing on your external monitor. It's not a deal killer, but also not ideal when working with clients whose eyes are glued to the external monitor and you are trying to shuttle through footage quickly. I don't know if this is a Premiere issue, thunderbolt limitation, or UltraStudio issue (or all 3). Black Magic says their in house system does the same thing. They point the finger at Premiere.
In either case, we're preparing to add more workstations and I'm curious if anyone is using the io XT with a recent iMac and PP CS6 and if you can confirm if it behaves better when shuttling quickly through footage or has the same results?