"but once the turbo spools, the supercharger does little other than get in the way of the compressed air."

Huh? The supercharger will take the compressed air, and compress it some more. The turbo's follow a pump curve that shows higher dicharge pressures = less flow volume. If the supercharger is to small, the turbos will pressure up until a flow rate matches the displacement / min of the positive displacment supercharger, or until it approaches the surge line and the recycle line opens to maintain minumum flow.