I think a maximum speed of 5-7mph would be reasonable on M.U.P's. I can admit to having ridden faster (up to 20mph), but only in straight-aways when no others were in sight. This does not make my actions right, and I would certainly understand if speed limits were imposed.
I would mostly agree with Jim I's description of M.U.P.'s as leisure parks.
I think what boy racers (like myself) need is a local velodrome, so that we can get our ya-ya's out, when needed.