On an all motor setup, backpressure is desirable because it helps maintain torque. If you run a 3" exhaust with all motor, you will lose not only HP but a lot of torque as well. Now with a turbo or supercharger you usually want as little backpressure as possible. A 3" then becomes a good idea. For your setup, stick between 2.25" and 2.5" for your exhaust for best results.
