how much power gain do u get this this?

I don't think that the power gain from the MAF by itself was anything to write home about, the seat of the pants test didn't show any gains.

With the other mods I had done I guess it probably helped.
Mods were: Inlet manifold upper and lower ported and polished, bigger throttle body, bigger injectors (not sure what size), extractors, exhaust, different fuel pressure reg, 3.45:1 LSD (to replace 3.23).

I guess the car made quite good power as I managed to break the input shaft on the T5 gearbox. (under warranty luckily).

unless the MAF is the right one for your injectors/ECU, all it'll do is change your air/fuel ratio to rich or lean.... with standard injectors, IIRC it'll run lean

To get around that you can replace the electronic module on the side with the one that is on your car. But I think that the MAF would make almost no difference by itself.

Interceptor: I agree I would think that the car would run too lean.