The T67 that often comes with the STS kits is a rather large turbo that produces "unfulfilling" results in the STS configuration. The Procharger does just as it should, belts up and blows air. In a traditional engine compartment turbo layout the T67 would school the procharger in area under the curve and overall performance. However with the choked down turbine A/R that typically would be used to compensate for a lower displacement engine or a twin turbo applications combined with the heat loss and travel distance of the hot pipes in the STS configuration makes it still produce admirable numbers, but not par with what the compressor is capable of from a power and response perspective.
Second, these cars a low to the ground, they are designed that way for cruising and "elevated" speeds. Having the lower stance tends to cut the air from out from underneath the car to improve stability. The "pipe" that is used to transport the compressed air from the turbo up to the intake doesn't take on the same airflow that the procharger intercooler would (which is a poor choice to compare an intercooler with that "pipe" anyway) and doesn't have the same heat transfer, ie efficiency capability, that the procharger would have in mounting the intercooler in the nose.
Finally, take a look at available air at speed. When you're driving down a dusty road, does dirt settle on the nose or on the tail? It settles on the tail because there is less airflow at that point to blow the dust away, that's right where STS put the air filter, the procharger filter is located in the engine bay near the fenderwell, which is right where GM designed the air inlet to be.
In this application, the Procharger, despite the fact of being belt driven, wins out. When the T67 is mounted in the engine compartment and given a proper front mount intercooler, the procharger will be left wanting, but for the time being it takes the cake.