I'll see if I can find a link to the article I read on the Prius/BMW comparison, it was a while ago.
Propane and hydrogen aren't synonymous of course, so a propane conversion isn't the same as a hydrogen conversion. There are a lot of con artists out there selling "hydrogen boosters" and "conversion kits" that connect up to your alternator and electrolyze water into hydrogen and oxygen to inject into the combustion process, but they're dubious at best.
A liquid hydrogen tank rupturing would immediately decompress to a gas and fill up a huge area. Yes it would rise somewhat quickly, but the largest chance for an explosion is in the few seconds following the accident, and given that it's a rapidly expanding gas, any nearby spark could ignite it. It is *at least* as dangerous as gasoline, if not more so, especially given that its contents is kept under pressure.
199mph is the record *for a hydrogen powered vehicle*. I'm not sure what your point is as the record for gasoline vehicles is over 400mph
and even non-specialized road-legal cars can go over 200mph (e.g. McLaren F1, Bugatti EB110, etc, etc.). So I think the point that hydrogen is a less efficient fuel holds pretty darn strong.