Equal numbers of T-80U will beat equal numbers of Leopard 2A4. At max range you deal 2 damage compared to his 1, have 10 RoF vs 7 and accuracy 8, which is less than Leo's 10, but still enough that you will hit often. The ATGM is not neglectable as it's a great addition: it's accurate, has the highest ATGM range possible and is SALH type with 8 accuracy.
And it's not hard to beat 4 leo2A4s with 2 T-80U + support. I personally find that in the current meta, 2 heavies are the optimal choice as it frees up money for other things. However, that's unfortunately the problem right now that you actually will NEED those 2 T-80U's to hold your own against the Leo2A4s and vice-versa. Anyway, this thread was about how heavy tanks are too much of a dominating presence in the game, not whether T-80U is OP or not (hint: it's not).
Well, obviously a complex matter here.
Anyway you said that Leo 2A4 is obvious because it outclass the rest of NATO tanks and it's very true because it has max points in key stats for a tank. If T-80U approach you just fall back and switch to stabs.
ATGM is weak for reasons I stated before. Also drain supplies.
So you field two MBTs, I for the most part too... for ranked mostly one. But when you got your Leos destroyed you have two more to field with 10 AP gun to engage 10 armor Pact tanks, when someone loose his T-80Us he is in some serious problems.
In hypothetical situation you don't need a heavy tanks to deal with another but it's just theory.