With a real Tony, he would be unlikely to go this far. With a real Tony, he would be more interested in sex than pain - the beating would probably be over by now, replaced by more pleasurable activities.

This is not a real Tony. (And Obadiah is curious about how much Sherlock will take, if he can live up to his confident claims and incredibly lax limits.)

He doesn't say a word, doesn't even touch Tony, before the belt comes down.