why'd you get a 3in downpipe with a 2.5in exhaust? you're not gonna make as much HP as you could be making as you're going from a larger diameter to a smaller diameter. that's gonna cause turbulence in your exhaust. on a boosted car, you want the exhaust to exit as fast as possible. so basically you either run open downpipe at a larger size or you run equal size exhaust all the way back. you should definitely avoid running larger pipe to smaller pipe like that. more or less, you're forcing a large volume of exhaust to go thru a smaller volume of pipe, which will cause more heat and less performance. hope this helps.