When looking to buy an exhaust, an of the shelf header I'd recommend all day long compared to a custom made one. The header is very important and tough part to get right when looking to upgrade, length of runners, distance to step, full merge, firing order to consider. As for the cat back, a custom exhaust builder can just copy the design of a Nismo, ark or whatever you want, it's just piping, silencers and flanges.