High flow/removing the cats adds about a solid 2 HP. But the purchase of long tubes nullifys the stock cats with the ORY (Off road Y Pipe) which has no cats. But if u were doing the true duals, you wouldn't need an ORY. basically if I had the money to mod my car I would buy (in no particular order) LS6 intake. Longtubes and a desired exhaust system Like an SLP dual dual or loudmouth. Put the mods on all at once and go grab a tune. Talk about power you would notice. These mods are all supporting mods for a heads, cam setup.