It requires flight test to know how a windmilling or stopped prop compares with idle prop and the results are likely to vary considerably for different aircraft and propeller types.
Very true! I've done some of this testing recently on a diesel conversion to a well known type. The prop change associated with the diesel installation changed the gliding characteristics. We're not finished the testing yet...
I changed from a two blade Hartzell to the three blade MT on my O-360 powered plane, and the gliding characteristics were changed considerably.
It is possible (though I speculate) that the Cardinal RG involved in this accident had a different propeller that original? I've seen a few three blade conversions on Cardinals. That could affect how the airplane glided, and would not be accounted for in the original POH. It is less common for prop change STC's to be accompanied with a flight manual supplement describing different glide, but it sure would be a good idea! And, as great an airplane as the Cardinal is, their wing airfoil is not the most ideal for low speed increased G maneuvering, it's just not what the Cardinal was designed to excel at...