High flow cat
 

In terms of hp. and torque, does anyone know how much power I would get from a high flow cat system on my protege 5 that already has a header and a cat back?

Catalic converters are there to burn off eccess gasses to prevent polution, make your car sound less louder, take away your horsepower, and in some instances of small japanese cars, catalic converters are there to keep a good back pressure to the engine which in turn keeps good pressure inside the engine (interesting huh?); thats why sometimes you hear people complain about slower car, lost horsepower and rough idle when they removed their stock exhaust. So, i would say if you put a cat back exhaust and you still have an original catalic converter, to go ahead and get a high flow cat. You will more then likely notice some deeper exhaust tones and maybe even some horses.

does your header have a test pipe?
if so you've already removed one of your cats.
a high flow cat is usally empty or has very very little in it
so you'll be better off taking your cat and drilling holes though it or just removing it all togther you'll be good aslong as you keep you resonator.