well 1st issue is the celica aftermarket header, they are essentually all crap. For the gts they can even loose power. Your best bet is either the new PPE header (i think the gt one is finished maybe) or have boosted2.0 port your stock one. the ported stock will give you alittle more low end where the ppe gives you alittle more high end. these can be found over on newcelica.org under performance. the ppe is around 500 and for boosted to port your stock header its around 150 i think

as for hte long cylinder, its either the resonator or muffler. cutting it off will make the car louder and be abit more free flowing. on a NA non boosted car you dont want completely free flowing as some back preasure is needed.

Though i'm still not sure what you mean by cutting it off and giving you a catback. A catback exhsut is just that, goes from the cat to the tip. all celica exhausts with the exeption of the TRD kazooma (essentually strait pipe) are all axel back because they bolt to the stock pipe at that point. Cat back for the celica requires cutting off at the point after the cat and welding on a new exhaust.

I hope that all helped and i wasnt too long winded