As a passenger I have gone as fast as somewhere in the 160's. This was on the back of a GSXR 1k which is one of the most uncomfortable bikes to ride on as a passenger, IMO.
I love speed and I love twisties. I don't think I'd ever have the figurative balls to do as I rider what I have done as a passenger. Which is the opposite of how most people feel, I know. And also part of the reason I'm a perpetual passenger.