I too was looking at these and from the research I have done the potential gain is indeed quite small if anything. The stock exhaust manifolds flow quite well, the restriction begins in the down pipes and continues from there. For a nice note and a marginal power increase a cat back system will do the job fine, for all but N/A race applications a custom DP and high flow cat would give you the best street performance.

Extractors may give you that old school sound but even at the exceptionally low cost I dont think they are worth it, but I am happy to be proven wrong