That's what I meant when I said "the CoP moves with rudder angle" -- and also with prop rpm.
The difference is nothing to do with speed, it will happen when manoeuvring or cruising slowly at a few mph -- but given that your rudder has a relatively fine profile (5%?) the difference won't be as big as with mine (20%).
If you want to actually measure the before-and-after difference you need three spring balances, one attached to a rope holding the stern back to something solid (thrust), one attached to a rope running sideways (steering), and one to pull the tiller sideways (steering effort) -- this is how rudders are measured. With the engine rpm constant, measure the three forces (thrust, steering, tiller) vs. rudder angle. You should find the new one gives more sideways force (steering thrust) at a given angle than the flat plate, and carries on working to bigger angles, but might also be heavier to steer -- or maybe the tiller force needed for a given steering effect is the same but you don't have to push it over so far.
This would give you some actual measurements about how much better it works, as opposed to subjective "it feels better" which is always prone to expectation bias (result being what you expected).
I'm hoping to try this with Ricky when my boat is built, if there's another similar hybrid boat with a flat plate rudder in the water at the same time...