So the same core machine with more HP would be better for you.
Think of it as a bunch of subsystems: sickle, feederhouse, rotor, cleaning, clean grain elevator, etc. You can only go up to the level where you hit the limit of any one of those systems. If you're currently hitting the HP limit, and you increase the HP, then you'll be limited by whatever is the next limiting factor. More HP means you'll drive faster, so it may be more of a challenge to get the crop through the rotor and sieves with minimal losses, since you'll be pushing more crop through the same rotor.