I'm using Cubase 5.5.2 on a MacPro, older quad 2.66ghz. Using vstis as plugins was
quite problematic, no hope of workably low latency
before VE Pro networked to the slave octocore. My current project is pretty loaded and my buffers (RME Multiface 2) give me a reading of combined I/O
~11ms latency, using the x2 setting in VE Pro and given the latency compensation it's not so bad.

A try at this kind of project would involve
a lot of freezing tracks and buffers always @ 1024; in fact such a
project would just be too problematic without this network handling.
Cubase is not the best idea on a Mac, but VE Pro has allowed me to keep
using Cubase, which is what I'm used to. 5.5 improved the multicore
handling significantly IME.