Until you do more with the delivery of fuel and cooling the intake air nothing more than the stock exhaust "straight pipe" will really do much good. In theory you want a vacuum on the exhaust of a turbodiesel, but the curve of gains by size increase gets flat quickly. The modified stock IP with up to 16psi stock turbo and close to that with a VNT needs little if any more than stock "SP". Going from stock SP to a 3 inch SP on the few that reported it showed little improvement unless you had a finnish IP or other fuel mods.