When the throttle isn't wide open the carb size is irrelivent as the throttle is the limiting factor.
You can undresize the carb to act as a limiter, IIRC there used to be a comercially available limeter that did the same.
If it were my engine however, I would prefer to set the wastegate to limit the boost to a reasonable level for a given engine, rather than choke off the turbo.
As for the shop vac speeding up, that's because the vacuum impeller is not driven by any function of the airflow coming in the hose (being driven by an electric motor it will run as long as current is available).
