Still that is power creep, even though you pay for it. Imagine if pz4 could pay side armor skirts, tungsten rounds for extra pen and he rounds like Sherman's. It would be insanely OP
Armour skirts are a bad example, most Ostheer tanks already conjure them with veterancy.
But true, I don't much like the idea of a T-34/76 upgrading into a KV-1, and this is pretty much the same thing.
I think it should at least cost fuel and be mutually exclusive with the .50 cal.
Ideally, ditch it entirely and add some sort of Sherman Jumbo unit instead.