Based on your other parts 2.25" is fine. But 2.5" is proving to be the real power maker. You should consider a 2.5" collector header and 2.5" exhaust (Custom since 63mm is the largest brand name size I know of before 3").

A test pipe won't make more than 1 to maybe and it's a big maybe 2 hp over a high flow cat. If any power at all. the cost is the true justification.