basically all anyone is trying to tell you is get a 2.5 inch exhaust system, it will improve your power curve and make it sound a lot meaner, if you have a 3inch exhaust you are wasting money without having a turbo, supercharger, or running nitrous. and since you already have 2.5 inch header, and downpipe you are just causing back pressure by making the rest of the exhaust an extra half inch, trust us man get that exhaust more free flowing and your engine will respond much better even you will be surprised, if you don't want to take the risk of spending that extra money than do this, unhook your exhaust from the downpipe so now you only have the headers and downpipe hooked up (maybe even your cat I don't know how your pipes bolt up) now go for a little drive, you will feel the same difference but not quite as loud as you would with 2.5 inch exhaust the rest of the way. If you want 13's then either take our advice or spend 3 grand on a turbo kit cuz thats what it will take.