Man...im disagreeing with everyone on this forum today! I could go with the cat before the plugs. Those dual cats you have strapped on your ride are very restrictive. From personal experience, when I installed my flowmaster, I saw (felt) no gain and developed a horrible sound coming from the exhaust... I took it in.. and it turns out the cats were causing to much restriction and the muff wasnt getting enought pressure to even sound right. So, off came the 2 cats, on came a single high flo, and now my muffler sounds great and a little more power to boot. (and I do mean little!)
nick