How much would a drop from a 3 diameter pipe down to a 2.5 inch diameter pipe hurt hp? This section would be 20 inches long located just past shift linkage. Everything else is 3 inch diameter from the turbo to that point and past. The car is approx 400whp.

 

Why not just run 3" the whole way? You're going to have a bottleneck effect through that section which is going to limit the amount of exhaust flow. I'm not an engineer and i've never taken a fluid dynamics class so i can't shed a whole lot of light on this in a technical way, but i could only assume it would hurt performance. At that power level, i dont know how severe of a limitation it would be...i'm sure there's someone here who can give you a definite answer