ok, got it. I don't think leaving the dual cats and going back to a single Y and 3" tailpipe will affect your mileage nor performance. However, if those downpipes and Y are not mandrel bent, that could make a a difference. The whole idea of performance and efficiency is to get the exhaust gases away from the heads as quick as possible. Mandel bends, headers, Crossover etc up in the front of the system all have the biggest effect. As you get further back, it's much less important unless you really restrict the back.
