X2, but the major reason it doesn't always produce the expected results is that the power system (enrichment for high load) has not been re-tuned, and is brought on-line prematurely.
This is not the same as the actual metering device (jet, metering rod step, PVCR), but the point where they are initiated.
You typically need a lower PV point for Holleys (4.5 instead of 7, etc.), and a weaker spring for metering rod carbs (AFB, QJ).
Whatever vacuum level you have at normal cruising, the PV must be at least 1" (IMHO) below the indicated vacuum and closed, and the economy rod step still in the jet (the "down" position).
