I had one fitted to my falcon not too long ago, I just had mine dynod on the weekend.
Valve open: 198.9rwkw
Closed : 138.0rwkw
When open there was no restriction at all compared to my previous straight through muffler. This was with a full 3" system and a very big cam.
They feel very gutless when closed. I would only use it when you really need to be quiet.
Iv seen some decent power out of some NA cars on this forum, surely there are decent cams for the engine. Do air intake, extractors, exhaust, cams and a tune. Any further and you will be spending big bucks