You can manipulate many things on a Mustang dyno to get any kind of # you want. It doesn't mean it's right. You can't do that on a DynoJet.
Mustang dyno's consistently read ~8-10% lower than Dynojets, and that is a proven fact.
So, you see these low #'s on a MustangDyno, and try to equate it to BHP, of course you're going to be seeing a huge loss through the drivetrain, but that is not correct.
Vipers do NOT lose 25% through the drivetrain, come on, that's more than an average AWD loses (~22%). Sure, that's how much you have to correct for on a Mustang dyno to get what you want to see in BHP, but it is not a correct indication of how much power is actually being lost through the drivetrain.
15% is what the majority of people use for RWD cars, and it seems to be accurate.