420A 2.5 inch exhaust with 2.5 inch headers?

[Project2GNTCF]

I am looking to buy an ebay exhaust and I dont know whether to buy 2.5 inch exhaust with 2.5 inch headers or 2.5 with 2 inch headers? Which will give overall better flow as I do not fully understand exhaust gases leaving the system

 

[TheBassist]

I'm sure 2.5" all the way through would be just fine. I have a 4-1 header that has a 2.25 outlet and I'm about to put on a 2.5" catback. So I think 2.5" would be fine.

 

[Haase]

2.5 inch is the commonly accepted ideal size for n/t cars, so i would go 2.5 inch for the everything including the header