Al.Fisherman,
I have 3 MB's & the fastest one is also the heaviest one (DH MTB full suspension). It can hit 55 mph with me in an aero position (aka-lowracer) & is as stable & refined as a small motorcycle. My lightest MB has no suspension at all, but the same engine & could probably do the same speed, however I'm not crazy enough to risk it on that bike since it doesn't feel safe at high speed.
-Lowracer-