I've not noticed any performance changes in the few VMs that I upgraded to v7 although I've not done any scientific testing. Anything new stays at v4 and works just fine. These are mostly Windows guests - both server and desktop OSs.
What I can tell you is that the VI Client won't allow you to edit the settings of v7 VMs so you have to make any changes to these in the WebUI. This has been the deciding factor for me to stay at v4 currently unless someone comes back raving, with proof, of far superior performance and/or density.