FWIW, I have an 8-core Mac Pro. It's completely hamstrung by the requirements of awful, expensive FB-DIMMs. On anything that cares about memory bandwidth (like pretty much everything) it IS trounced.
Not that I'm complaining - it is after all 12 months old. When the new 'Pros are out, I'm sure they'll leapfrog again. But I was hoping I could, get some iMacs with, ideally Core i7, but at a push quad cores for the office. These alleged "updates" seem pretty stale to me. I'm prepared to pay a significant premium - but there's no way I could justify the kind of price differential to a Mac Pro which, in the short term, is slower and twice the price; and in the long term will give performance we don't need and be twice the price.