why are you chasing gains in the zed? Its difficult to get decent hp to cost with these cars.
maybe an evo, supra, 300zx, rx7 etc etc would have been a better idea?
the system measures the difference in wheel speed between the 4 corners so if the speeds are corrected then the system will work again. essentially the system isn't broken, just being given incorrect info.
i would get some good tyres then maybe some good brake discs and pads if you want to faster round a track. the zed is the wrong car to go bhp hunting unless you have that 8k lying about the mechanic talks about.
as for the mechanics attitude i would make sure everyone i knew avoided that garage, pathetic attitude.