You can attach a physical disk with Hyper-V, so that it is "owned" by that particular VM and no other. I don't have Windows  Virtual PC, so I can't confirm whether this feature has been back-ported. (I suspect not.)
This feature is also supported by VirtualBox and VMWare, using a special .vmdk image file (which just points to the disk).
The performance benefit may not be all that great, but I suppose it's worth testing in your particular setup. The phrases "raw disk" and "disk pass-through" are used to describe this feature, if you want to search further.