Alright mate,
There could be a few causes of the power loss. First, because you have ditched the air box you are now letting in the hot air from the engine. This significantly reduces power, see if you can get hold of a heat shield. Secondly, it may be worth getting the fuel/air mixture checked becuase now you have more air going in to the engine.
It might also be worth checking if the kit is fiited correctly and that it is the right kit for the car. My mate has got a VR6 and he ordered a jetex induction kit, the car pulled like a milk float, then we found out they had supplied the wrong size cone filter.
Hope this helps
Tom