Great idea on the exhaust but hp gains will be minimal if any unless you get a header and high flow cat to go with it,only because your stock header and cat won't be able to flow anymore than they already are.Idealy you want same diameter pipping from the collect to the cat for optimal flow and then you could possibly reduce pipe diameter to what you want if you want. If I'm not mistaken you could actually lose some hp because stock exhaust systems are desinged for optimal backpressure and flow going to bigger diamiter pipping after the cat may hinder pressure build up in the bigger pipping resulting in a loss of power.Do the whole system and you'll be doing great.Or bump up your hp to increase flow.
