Originally Posted by suss6052
The 2.5" would have a lower restriction than the 2.25" tubing, however if you go too large you reduce the backpressure too far and start to hurt performance.
Its been discussed that the 2.5" might be a better plan if you are planning on adding additional power adders down the line. If the limit of your modding is an intake and exhaust and nothing else a 2.25" exhaust may work better.
Back pressue is not a factor in direct injection engines, as fuel charge scavenging can not happen, becasue the fuel charge is delivered at full compresion.