My crazy brother Steve used to drag race motorcycles professionally. He wasn't in the top unlimited classes, but he routinely went from 0 to 160 MPH in a quarter mile. He said the wind and acceleration forces were pretty strong. He told me of a older rider who rode the unlimited class bikes that could exceed 200 MPH. That guy was blown off his bike during a race and killed.

The drag at 400 MPH would be 4 times as much as at 200 MPH, so the rider would probably need some restraining device to stay on the bike. Even then, it probably wouldn't be a very fun ride. It'd be my luck that I'd hit a June bug (a big flying beetle with a hard shell, shown roughly actual size on this page) at that speed. It'd be all over.