Snow leopard is a 64 bit OS as well..so no performance hit,
Avid runs on Snow Leopard with good performance, but it is not “supported”.
All Mac Pros have 64 bit Xeon processors and run 64 bit apps with all benefits provided you use Snow Leopard or Lion. The first two models of The Mac Pro have 32 bit EFI and do not run the 64 bit kernel, but that does not affect running 64 bit apps in 64 bit mode (this differs from Windows, which cannot do this)…only affects how the OS runs kernel extensions to talk to hardware cards and other devices. All Mac Pros from 2008 onward can run the 64 bit kernel in Snow Leopard or Lion.