[Marcello]

I have thought more about it. If the 175mm gun is meant for a tank destroyer, rather than some uber tank as I assumed initially, it might be possible to set up something to load it without an autoloader. The shell for the Sturmtiger was 345kg so handling a 175mm round should be technically feasible,if still massively unwieldy. Still it isn't a good idea; you have to remember that in real life the germans were cut off from tungsten sources when Franco turned the back to them. Given that all those advanced shell designs required tungsten this meant that they could not be produced. Therefore the only way to improve AP performance was building bigger guns, despite the disadvantages.
Assuming that instead tungsten can be purchased it seems to me quite likely that the germans would have developed more sophisticated rounds, it is not like they were clueless in this area (see the PAK 41 for example). As I said a 128mm with an APDS would deal with everything the russian can field, save maybe (and I mean maybe) the few IS-7 which will be made. For those a few 150mm with APDS will be more than enough.