It shouldn't make much difference lower down in the rev range, but i'd think it would really hamper the performance higher up in the rev range as LPG doesn't burn as fast as petrol.
LPG is really suited to larger capacity lazy engines, rather than the rev happy nature of say the M5. The 540i wouldn't be a bad conversion (in fact, given the price i've seen a couple of 540i's go for recently, that may be an option next year!) but i wouldn't do it to an M5
Incidentally, a while back my mate's dad had an otherwise standard E38 740i (4.4 litre) on a dyno... It made exactly the quoted 286bhp on petrol, but running on LPG it only lost 2bhp!
What would be interesting though is if you knew what you were doing, or knew someone who knew what they were doing, getting the car mapped with 2 different ignition maps (one for petrol and one for LPG) so that you can advance the ignition loads for when it's on LPG. Whilst it takes longer to burn, LPG has a higher Octane rating (typically 10 points or so higher) than petrol, so for a large capacity low revving engine, it could be used as a performance fuel! Especially if the compression ratio was upped a little bit too...