OK, just tried one of these at our campus bookstore yesterday. They had Aperture on it and I loaded a Nikon D600 (24 mp) RAW file and the computer handled edits in real-time. I was kind of surprised - no lags or stuttering at all. I understand Aperture uses the gpu a lot so the HD5000 could account for it. They let me download and install Intel's cpu monitoring application and when I was using the blur brush in Aperture the clock frequency ramped up to 2.7 ghz almost immediately. When the computer was idle it dropped down to about 800 mhz. My understanding is that when the turbo kicks in it shuts down one of the cores, so this is probably just a single core running at 2.7 ghz.
I don't know how it will do with other things, but is seems like this new iMac is a much better computer than many people think - including myself.
Good test, thanks for sharing.