OSX SMP speed depends on the applicaion
First Mac OSX needs to efficiently use 2 or more processors (that's pretty much done), then the applications developers need to tune their applications to take advantage of multiple processors on OSX. I think that Photoshop is tuned for OSX, but I'm not sure. I expect that by the Fall there will be a pretty long list of apps that take advantage of multiple processors under OSX. I think that Apple's web server apps will take advantage of multiple processors right now.