They also rob horsepower apparently! Although have thought so on a turbocharged engine. Supersprint are also very good as they are tuned to the car specifically & a few people on the forum recommend blue flame & there were some for the coupe 20vt on ebay recently.
I doubt any aftermarket exhaust would rob power, especially on forced aspiration engines where removing exhaust gas as fast as possible is beneficial for more horses.
A less restrictive exhaust is more important to turbocharged engines than for N/A ones. The turbo uses the exhaust gas from the manifold, but of course using a turbo means more gas to remove