[LUKassZ32]

Still not knowing a lot about these cars, or their stock A/F ratio...do they tend to run a bit lean or a bit rich?
But just a thought, maybe with the 3.1 inch pulley you were running on the lean side, and now your a/f ratio is a bit better with the bigger injectors, even w/out the tune.

Granted this does not explain the increase in power, because the leaner you go, the more power you get.