Yak-9T - incredible firepower (or maybe incredible tank's lack of armor?)

[gaunt1]

Perhaps even 20mm AP. Depending on the exact shell type and striking angle, 20mm AP could penetrate up to 60 mm of armor plate:

Impossible with ordinary guns. Thats almost as powerful as a GAU-8 Avenger! The weapon you are referring to is a 28/20mm squeezebore gun, with very special ammunition.

http://en.wikipedia.org/wiki/2.8_cm_sPzB_41

Check the other guns on that site. They are using much larger cartridge than aircraft guns, and their barrel lenght is also significantly longer. (except the Breda).
Even modern 20mm guns are unable to penetrate more than 40-45mm armor:
20mm APDS DM-63 44mm at 1km (1988 )
Bottom of this page: http://collinsj.tripod.com/protect.htm

WW2 20mm guns could penetrate around 10-15mm armor, which is not enough against tanks due to the dive angle.
A Tiger, KV-1, IS-2 should be completely invulnerable, only the 75mm could kill them.

Also, from wikipedia:

In spite of the large round, the VYa-23 proved to be a disappointment in its intended anti-tank role. Light German tanks could be defeated from the side or rear only, with front armor of all tanks impervious. Medium tanks could be defeated if hit into the top of the turret or the engine compartment from under 400 m (1,300 ft) in a greater than 40-degree dive -- a very difficult maneuvre in Il-2 even under the most ideal conditions compounded by the difficulty of aiming at a small target.

And the Vya was far more powerful than any 20mm in the war!