first cubase does not utilize hyperthreading. second your machine is a version 2,1 which is an older model and Im not sure what has changed in the architecture recently but w Mac Pro we are at least on version 5,1 intel hardware now
You can test your project by deselecting multi processing in the vst audio device panel. Some pluggins that do not properly support multiprocessors you may see an improvent in performance. for example i had some older waves version I was running on C4 that i just upgraded from waveshell 6 to version 9 and the new versions are noticeably more efficient. Also 2c Audio 's reverbs are heavy duty computationally.
FWIW i am just moving into 64 bit mode and that is helping me - i just inquired recently at a post (link below) and while it that info was more specific to windows there are a few posts that should shed some light on the problem.
Have a look through this thread and check my post at the end
HTH
phi