Tifa would definetly win because she is way faster that Ryu and is still quite powerful. You say that Ryu would have about 20160 hp and Tifa's max is only 9999 but what is Ryu's max damage? i've gotten Tifa up to 9999 in a regular attack so that means Ryu is dead in 3 hits.
If Ryu has that much HP what the hell makes you think his atack would be as low as Tifas.