advice needed on exhaust, urgent advice on exhaust! Options

[krayziekurupter]

hey whats up everyone. right now im riding with open headers that i just installed and i need advice on what i should do for my exhaust. im wondering what would be best, like if i should leave my regular diameter of my piping or get 2.5? and if i should keep my old cat or get a high flow cat(would it make a diff?). really need some opinions on what would be best to gain some horse power. thanks

[krayziekurupter]

my state doesn't require to have a cat but the reason why i wanted a high flow cat was because i hear around that 4 cylinders need back pressure or else i'll lose some horse power.. and for the next thing and last thing i'll do is a performance chip to the engine

[presure2]

my state doesn't require to have a cat but the reason why i wanted a high flow cat was because i hear around that 4 cylinders need back pressure or else i'll lose some horse power.. and for the next thing and last thing i'll do is a performance chip to the engine

that is a myth.
there are no "chips" for our ECUs, that work.