How much of an impact does the performance of the storage system hosting the paging files of windows machines (I guess both physical or virtual instances like VMWare / KVM / Xen / Whatever)?
I can only speak for VMWare products (and you don't mention a particular product of theirs - perhaps you can). But certainly with ESX/ESXi 4.1 I'd be strongly tempted to allocate more virtual memory to your VMs than you would do and care less about the swapfile - simply because 4.1 is so good at managing physical memory (with shared and compressed pages for instance) that you may actually reduce the need to swap by quite a margin, meaning actual swap speed becomes less important. I know Hyper-V has very recently moved a little way towards in this area but I can't speak for Xen sorry.
As for the actual question; think of it in physical terms, do you put your swapfile on a separate high performance drive now? if you do then perhaps you need to look into your application and/or memory setup - but certainly a faster swap drive will obviously help in scenarios where you're swapping a lot - and nothing about that changes in a virtualised world.
Hope this helps.