I killed a Tiger with 2 500lbers from an A-20 yesterday... But that is the exception rather than the rule. For me anyway, tigers are usually indestructible and made out of titanium.
The IL-2 guns are somewhat effective for me. I was able to knock out the engines of numerous Panzers and M4's during a GV raid yesterday, but I had to come in below treetop level, in the nape of the earth and wait till i was 400yds or closer away, and at a perfect 90 degrees to the side of the tank before firing. Even then it took 2 or 3 passes before results.
The only beef I have is with the flaks. An F6 or 110G can take out my turrent in 2 or 3 pings, while i pummel a flak with the 23mm AP from an IL2 and does nothing. Solid hit sprites and flat angles and all. Probably "lag" or some other computer voodoo thats never on my side in that situation.