[Imp]

I only had a quick look for real life stats Which look about right for the gun, did not look up stats for L7 iwhich in game is only marginaly more effective than the 20 pounder.
At your quoted range of 1800m L7s penetration is only +1 on the 20.

Quick look also T54/55 armor looks right the angles already factored into the game stats. Armour was roughly 100 at 60 degrees which is 20 as represented in game.

So yes I would expect the 20 pounder to kill a T54/55 easily at close range, 1800m the oucome will depend on how lucky your die roll was the result could be kill damage or no effect.