after many tests on my i7 the actual render(without loading of testures) takes the same amount of time on p8 and Pro2010, i have been told from numerous sources that if you have a large scene it will render quicker due to access to  larger amount of ram(memory).
i can only assume imac's will have the same results as they also use intel processors( which are all not true 64bit processors(so i have been told))
But i will have to say the whole Pro2010 software is better than P8 so its worth the upgrade in my opinion...

im rendering Pro2010 on an i7 860,8gb's ram on win7 64pro so my pro2010 is also running on 64bit

Getting old and still making "art" without soiling myself, now that's success.