I can tell you that the 3.4GHz i7 iMac works absolutely brilliantly with the Spectrasonics stuff. I went from a 2.4 Core2Duo MacBook Pro with 6GB to this specifically to get more headroom for Logic Pro. The MacBook Pro had done well with heavy Omnisphere patches but I had to freeze tracks in order to add in Stylus RMX and Trilian. Now I have virtually 8 cores instead of 2 so it cruises very comfortable with plenty of CPU headroom.
Also running the EvanLogicMulticoreBenchmark file, I ran 82 simultaneous tracks with the 3.4GHz iMac as compared to 20 with the 2.4GHz Core2Duo MacBook Pro--four times as powerful.