One instance of sculpture bringing your computer to it's knees, eh... well, due to the nature of the beast, and the fact that it is among the most CPU intensive plug-ins around, I'm not surprised.
How many notes are you holding, and how long is the release time? Pick a pad the has lots of decay, and play a 5 or 6 voice chord and watch the meltdown. I suggest you look at ES2 or any of the other VU's before you use sculpture. I think they designed the "freeze" function for that specific reason: so people could actually use the thing 8)
What is the cpu speed of your 8 core Mac Pro?
And the "8" cores in the iMac is a 4 core machine with 4 "hyper threaded" cores. Hyper threading is a way a computer can make 4 extra virtual cores, a method that allows more of the cpu to actually be used. So really, you have a 4 core system.
As for why only 6 cores seem to be going, I have no idea... BUT have you taken a look at the meters in the "activity monitor" utility? that is a much more accurate representation of what is really going on in your system regarding most anything: cores and use, ram available, when you are going too close to the edge with Logic in 32 bit mode (about 2 GB and the thing starts to act drunk), and the best secret in the Mac world: If you app freezes, open Activity monitor, and wait a few minutes. I have seen what I thought were completely lost songs on a frozen Logic become alive after a bit of waiting. An apple store tech told me that one, and it's saved my bacon a few times over the years.
Anyways, blabbing on, hope something I said or suggested helps...