2.5" mandrel is good, but a proper 3" system is always best and will see you get better figures. i went from original to JJR 3"- 3" stainless cat 175cell- HKS hi power silent 3", made noticable difference, but currently running split front pipe, 3" no cat and a straight pipe with HKS endpot. makes even bigger difference then with a proper system, sounds nicer too, but not a very wise choice! EPA, cops, etc
if yours is blocked, try swapping out the cat, if its not that, you could have a blocked Muffler, so to recap, only go to 2.5" if you dont plan on upgrading anything! go to 3" if you want more headroom