first of all, lol at "powerbox".ok, for exhaust, try and get a turbo back, as the dump pipes can be quite restrictive.
keep the cat, as long as you have a high flow, ahouldnt affect power.
pfc is probably there so they can squeeze more power out of factory set up.
twin plate would be for feel and longevity on a stock set up.
hope this helps.
intakes weren't restrictive apart from afm's at very high power.