Anything above 130 mph wheels up is risky, 200+ is suicide....
Think of a REAL plane "ditching" at that speed.... how would the pilot look after auch a 'landing'?
As Mustaine already posted, that "feature" (=bug) has been removed a few patches (if not versions) ago.
You can still land wheels up, but you gotta do it at reasonable speeds. If you absolutly NEED to do a high speed landing, touch down wheels *down*, then push the plane onto its nose/to the side, break off the wing and gear if possible. With a little practice, all this can be done staying on the runway.