If I put emissions Pacesetter Shorty Headers ($272.99) and a Flowmaster 3' single outlet cat-back ($259.95), Can I expect a 40-45 horsepower gain from the combination of the shorties and the Cat-back?

 

No, I would expect minimal gains. I didnt notice hardly any difference in power with my longtubes/dumped exhaust.

 

i dont know about 40-45 hp, especially not to the wheels. but freeing up the exhaust is one of the best things to do, and i'm sure you'll feel the difference. everyone will tell you how much better long tubes are, but i've heard the difference isn't that big. i'm totally satisfied with my mac mids and loudmouth catback. good luck!

edit: forgot i switched the loudmouth resonator for the borla xr-1 muffler lol

 

You will need a better exhaust setup and maybe a couple other minor bolt-ons to get the gains that you are hoping for.

 

longtubes x-pipe dumped and maybe a small tune up, plugs, wires, fluids etc could prolly get u 40-45rwhp but not with just shortys and a catback