Roger b

Im wondering if anyone have done a custom made exhaust. The reason im asking is because i got two magnaflow muflers as a gift, so what im planing on doing is a HFC, bigger DP and custom piping all the way to the mufflers. So heres my questions.

1 does a HFC make a real dif. Or should i just keep the oem

2 how big should i go with the piping? 2.5 or bigger? If biger how big?

3 do i use the same size pipes from the DP all the way back or what?

The car have a stage1+ apr (injectors , r1 dv ) carbonio intake and im not planing to put bigger turbo on it.

I wouldve bought a cat back exhaust kit but my buddy works At an exhaust shop and hes doing the work for almost nothing and got the mufflers for free.
Im all ears, and thanks for the replys in advance.
Roger