I got 535. I don't know how in the hell I did it, but I did do it. If you concentrate on hitting his head or feet, I think you get a faster speed.
I don't know how I got mine, which doesn't compare too well with Manny's, Fadeaway's, and HH's but I did it on accident. I think my optical mouse had something to do with it. edit: can't compete, I could only get 521mph before I gave up.