oh crap, im having the resonator removed (its what the guy suggested). oh well, guess i can always get a new system if it gets too annoying. it'll only have one muffler once it's done.
extractors are one of those mods where everyone is expecting a certain amount of difference, which leaves them open to disappointment when they jump on the dyno. Do you have any other engine mods? And remember that just because a fact sheet says your engine model will put out x amount of kilowatts, it doesnt mean yours actually does!
Hopefully someone will reply with the same model and extractors installed, then you can get a better idea of what to expect.
Regarding NRMA, I'm with them and all engine mods are ok provided you check with them first (because there are limitations on size,brand etc etc), and as long as its nothing too huge you should be right - I dunno how you'd go insuring a supercharged vehicle on your P's! :p. But ALWAYS ask and notify them first - once the work is done, no matter what it is - even just alloys, the current policy document you have becomes invaild until you get a new one that includes the work done.