>>/39674/
The gun the Leopard 1 has was made specifically to counter the T-55, I think the only time a Leopard 1 saw combat was with the Danes in Serbia and Canadians in Afghanistan so it's not possible to list examples of them coming against each other, however, tanks like the Centurion and M60 have fought T-55's and even later models using the same 105mm gun the Leopard 1 has and have done very well in doing so.
Stalinium... I actually don't see that as such an issue in Warthunder, not like people all ways say it is. I know it does happen though, the real IS-2 can only penetrate 150mm of steel, not 200mmm or whatever Warthunder says it can(although I do understand why they made it do this, the balancing would be awful otherwise, either it would be up against pretty much nothing it could penetrate or it would be fighting nothing that could penetrate it).