I just recently purchased a Macbook Pro 2011 13 inch with the I7 processor. As whim, I decided to try assigning 2 cores to my Win7 VM. Too my surprise, performance improved significantly and also did not drag down the OSX side. When I would do the same on my Core2Duo machine, everything would take a major performance hit, almost to the point of being unusable. So, the old adage of only assigning 1 core to a VM on a 2 core machine may be a thing of the past.