Yes, due to the different style DP's you're gonna need a touchup. As you've noted, it hasn't cut but there is some surging. The surging that you're feeling is the turbo dynamics pulling the extra boost that is being caused by the freer flowing setup you have now. The WGDC %'s that are needed by a car with a HKS DP are higher than those needed by a car with an APS setup. The problem is only exasperated by the sensitivity of the GMBCS (or was it a Prodrive...cant remember).