I bought 4-1 dc header for my 96 gsr i thought i ordered 2.5" collector but turns out when i recieved the item that it was 2.25 i forgot to ask the seller what the size was but yea any ways to late. So question is there a big difference? i have a 2.5" exhaust piping and im still using my catalyc converter

 

On a mild setup, there likely won't be a tremendous difference in gains between the two versions (and for the record, yours has a 2.00" collector, not 2.25"). The stock cat will be the major bottleneck in the system anyway.

 

whats considered mild setup? all i have right now is the header i just recieve, skunk 2 intake manifold, aem CAI, and greddy trust cat back exhaust sys. future plans is the rev valve springs retainers, and buddy club 3 cams, and i have type r loss motion asym.