I went w/ 3.5" dual turn downs ( no room to go over the dana w/ the stock gas tank), I doubt very seriously that h, or x pipe would show enough, "if any" improvement at over 600h.p. probly not worth their weight !
I disagree with you on that Bob. NASCAR Cup cars all have X pipe, they have for years now. If it did not add power and/or torque they would not be using them. Non restrictor plate Cup engines have over 850 hp.