Flackbait, 20mm cannon DOES kill Panzers...you must be near convergence and hit them from the side or the aft side, but if you do it,and if you are near enough, they are doomed. Believe me, I've tried it and it works.
30mm cannon is ok with tanks, ina G10 yesterday I got 2 panzer kills with only 40 30mm rounds. Again, go for the side and aft side. Anyway one of the kills was from the front-left quarter, and the tank went boom.
Try it, it works.