I have been running VMWare Fusion (4.0 now) for a long time, but off the main MacBook Pro hard drive.
Will I get faster overall performance of the VM/Mac if I put the VM on an external Firewire 800 drive?
In general, you could expect the Mac to get slightly faster and the VM to get slightly slower. However, depending on how much paging you were doing and how much disk access the VM does, it could be that the VM would get faster. The VM certainly won't get all that much slower, if any.