280rwhp / .85 = 329+/- engine hp
320rwhp / .85= 376+/- engine hp
theoretical 47 engine hp increase with a measured 40 hp rear wheel increase. I'd say chances are you have way more than 376 engine HP or you've done something to relduce friction from your drivetrain.
I say you gotta pretty good deal, but that's just my opinion...