There was a post over on Amsnow (believe it was BigAl), and he said the F-Cat speedo's were amazingly accurate. Comparing it to the standard analog speedo's on other sleds (including ZR's), it was very close to what the radar had posted ... sounds like within 1-2 mph. The older analog speedo's were something like 7-10 mph off.
So, if you're concerned with what the digital speedo displays ... it looks like they're very close to correct.
Hope that helps ^_^