Does the car boost at all? Does it just boost a lot lower than before, or does it not boost at all? There might be a problem with your wastegate, Deltagate wastegates seem to always have some sort of trouble. (i think that's the wastegate that comes with the drag kit). Perhaps the wastegate is opening way too early for some reason? Check the vaccum hose that connects to the lower "boost" port on the wastegate, make sure it isn't kinked, off, cracked, etc... You may also have a leak somewhere, say at a connector boot between pipes, that can bleed boost also. Also, maybe check the blow off valve, perhaps it's staying open, and bleeding off boost for some reason.

Lastly, do you notice the car pull as if it were boosting, but the boost gauge isn't reading boost? If so, then the problem wouldn't be with your motor, but something simple to do with the vaccum line between the motor and the gauge.

hope some of this helps.....