However, how much improvement in horsepower do I need to fly stunt and sport?
None. Which is why, if you read various posts on engines you see
over and over again that raw horsepower is not what is important -- it's how good of a "stunt run" the engine delivers. In fact
here is a post in one of your own threads that gives a short definition of a good stunt run -- and it, plus this thread, proves that you've been told and
have not listened.
If
any of the engines that have been used for stunt since 1960 or so starts delivering close to maximum power, we say "the engine ran away" and probably consider that the flight was ruined (if not the engine, too, for some engines).
What needs to be good is a tractable engine that matches the airframe (or better, that can be easily tuned to match the airframe), that provides power that varies according to the maneuvers being done, and that does so the same way time and time again. An engine that takes forever to start and then delivers a fiery 80MPH flight one run, and then barely slobbers through a 40MPH flight the next time with no or minute changes in adjustments does not provide a good stunt run -- but it certainly managed to provide lots more horsepower than Brett's reliable PA (I think it's a PA) or my 46LA does during a good stunt run.
The reason that the stunt community has settled on just a few engines is because the requirements of stunt are peculiar, aren't addressed well by "normal" RC engines, and because the way that you get that desired performance out of any given engine design is different from the next one. So when some newbie/sport flyer comes on line asking about a bazillion different engines of varying age and conditions, the experienced hands all pile on and tell him to get just one PA or just two 46LAs, or maybe a Stalker, and
stop screwing around and just fly. Because otherwise you're just repeating experiments done long ago on engines that didn't make the cut, and you're not getting out there and learning to fly stunt.
If you want to mess with engines and you only understand horsepower, fly speed.