May I just say something very few people ever think about? What the factory considers good performance and what the average tuner considers good performance are completley different things.
A factory header will look for a particular power band, whereas most tuners look at the end of a dyno chart. Guys on this forum do this ALL the time. "Hey how mmany HP did you gain?". 10 HP between 7000 and 7500 RPM's may actually make the car run SLOWER than 5 HP that is gained at 3000 RPM followed by a fall further up.
The Basch Boost superchargers I have seen make over 400 HP whereas the Comptech autorotor makes closer to 350. The autorotor cars are faster.
Other than my GT-1 header, the Fujitsubo header was the most expensive header on the market. When Vancehu put his car on the dyno, 5 HP was gained at the top, torque was lost down low. But all anybody ever talks about is "this part makes 10 more HP". That doesn't really mean much. The issue is that the aftermarket people want big numbers so they can sell their stuff. The buyer wants big #'s, because well... a number is easy to understand.
I am also EXTREMELY weary of an aftermarket company that makes a product, then dyno tests it themselves, and finds gains. Guys... I've been involved directly with some aftermarket tuning companies. What everyone thinks is some scientific test with a dyno sheet as proof, is nothing but 2 guys standing in a dyno shop, one of whom makes a product he wants to sell, the other who is his buddy. Even when they don't inentionaly want to lie or mislead, this is what happens. "Oh we lost power? must be something wrong with the data. Lets re-test". Or "maybe the engine is hot, bring over that fan, lets sit for 2 hours". Enough tests are done until the desired figure is found, which even the part manufacturer feels is the "real" number.
I ran my car on 5 dynos, same engine, same gas, same similar outdoor temp, and got 318, 340, 350, 368, and 395. How can it be 318 in one place, and 395 at another? Nowadays everyone seems to test their engine on the "heartbreak dyno."
Whatever....
I think to try and get an accurate gauge on a part that makes a small HP difference is almost impossible. I can run my engine on the same dyno RIGHT NOW, 5 times, and get 5 different numbers. Yet someone makes a header, and says "this nets 16.5 HP". Truth is, no one knows what kind of difference it makes. But as long as people have cash in their wallet for a part, these numbers will appear.
Perhaps I am jaded, but I don't put stock in any of this anymore. Unless a factory race team, a car manufacturer, or someone with some SERIOUS knowledge, facilities, money, and engineering capability tells me numbers, I just take it all with a GIANT grain of salt.
I just changed my GT-one 5.0 exhaust for an ARC. The piping on the ARC is slightly bigger. Is that better on a supercharged engine? I have no idea.