So ok say a mild steel system with 2.5inch down pipe and then a high flow cat and 2.5inch piping for the rest with a custom muffler would be a pretty good system sound wise and performance wise?...for a cheaper option?

 

I would spend the money on the muffler and good piping installed by a professional to avoid leaks. I agree with Doose in that a high-flow cat will likely just cost you money with very little in the way of performance increase.