If anybody is looking for a great track tire (not applicable for street riding) that will fit on the stock GS 500 rims, Bridgestone makes a Super Sport Radial in a 110/70/17 and 140/70/17 that fits the stock rims perfectly. They are super sticky, and allow you to get all the way to the edge of the tire.
BT090
But you all probably know this already.