In my way of thought, necessary backpressure is a myth. Making the engine pump exhaust out with the pistons against a restriction just wastes power that could be used elsewhere. I know my car runs better with open headers than through 2-1/2 inch duals. 1-1/2 tube X 2-1/2 inch collector headers or magnum manifolds (Magnums for automatic or hydraulic clutch application) would be a good move. Are you having to pass sniffer test on emissions? Have you put manual hubs in place of the outer couplers so you can fully disconnect the front axle instead of just the left side via the CAD (centrally activated disconnect)? You can lock still your hubs manually and use the 4WD on demand as needed, but not have to run the right side axle and spider gears all the time when you don't plan on using 4WD. If you could get by with a high flow converter, I'd look at running a single 3-1/2 system.
