3 inch to 2.5 inch is going to create back pressure. Back pressure + boost do not get along. You'll lose power. You want to get exhaust out of the car as soon as possible. I would honestly recommend a 3 inch exhaust, but if you want to keep your exhaust 2.5", go with a 2.5" DP and charge piping.My exhaust is 2.5.. why is that bad? And he quoted me about $175-ish