I contacted Abbey Motorsport about an aftermarket ECU for the Z. They are trying to push the HKS F-Con at the moment. The final cost that they quoted me was well over £3k, Ive still got the quote that they sent somewhere so ill try and dig it out.
My car was dynoed a few months back & made just above standard power figures & well below standard TQ figures - this was with filter,plenum,exhaust,y-pipe&cats. I was having a few problems with the cats at the time though so im hoping that now I have my new cats in my torque will return.
To get the most out of N/A mods you really need an aftermarket ECU installed but at the price from Abbey + the cost of mods I would have been better off installing a supercharger.