I'd be willing to bet it is easier to completely destroy a Wirblewind via an eight gun P47 than it is using 20mm HE rounds. Not that it cant be done, but I've only ever damaged the turret and tracks of *any* armored AFV, I've not destroyed anything completely with 20mm. With the eight gun P47, I have been able to defeat the top rear armor of a Panzer IV but I think conditions were as perfect as it gets. I was in an almost vertical dive (perpendicular impact for projectiles), my speed was rather slow all things considering, the Panzer IV H was parked, and I was able to hammer home many shots right at the convergence point dead center of the top rear armor. Boom!
Otherwise, it is easy to damage the turrets of anything that has an open top. The M18, Wirby, Osty, M16, etc. They are are target fodder for aircraft. I wish HTC would tilt things a wee bit more in favor of the gv's in terms of damaging them via aircraft guns. I think it is way too easy. Just sayin'