My CZ20 is still a C20. It stopped ground balancing correctly and lost a lot of depth when the ground balance failed, so it is sitting in the closet. I have not had it converted because it is tuned to have nickles sound mid-tone so that I can just dig the mid-tones at the beach. Fisher will not tune them that way; Tom D doesn't tune the waterproof ones and I haven't talked to the guy who tuned my CZ20 in several years so I don't know if he still tunes them. I'm well equipped with water detectors so I kind of don't care right now. I suspect that ones converted to CZ21 will be just like any other CZ20 or CZ21 with some variance in performance from unit to unit but generally all performing alike.
Sorry I'm not in the position to do the exact comparison you asked for.
