You can observe the improvement easily. Pick a speed, say 30 MPH, with the plate fully down. Then raise the plate and you'll pick up several MPH due to reduced drag.
Based on the notion that boats are not designed with a wake plate, I usually go full up for cruising, and apply it downward to halt any porpoising. Compared to a couple of other brands I've cruised with, my Outback V will porpoise like crazy on rough water.