My 2 cents,
Flat is not fast, the boat likes 12 degrees not more not less. The problem is with the hull design of the boat. If you look at the hull with the boat on the trailer the boat is fairly straight from the toe rail to below the waterline, especially at max beam.
If we sail the boat flat you're pushing both sides of the hull thru the water vs. 12 degrees of heel your pushing just one.
I will agree that any more heel and you start losing the efficiency of the keel. The reason is one feature of the Capri 22 is the interior volume of the boat, being the max beam is close to the waterline. The boat starts to lift up out the water and the boat decreases draft and the slips to leeward. You can see this happening while sailing, look at the transom and the rudder you'll see the waterline go to leeward of the rudder as the boat heels past 15 degrees.
With regards to the wing keel, your biggest issue is fore and aft issue. The boat inherently sails bow up due to the fullness of the boat's volume forward. By keeping weight forward keeping the bow down keeps the angle of attack at the wings from not dragging.