Where exactly did you get this "very poor" mobility from?
Just because wikipedia states that the pershing had a very slow off-road speed doesn't mean it's true.
The reason why wikipedia gives just a poor off road mobility is because not all off road terrain is equal.
If you look at this video this gives a much clearer picture of how tanks actually performed off-road in ww2. (It's not very fast)
https://www.youtube.com/watch?v=SmXEly5_u38
The Pershing was underpowered like the Tiger tanks were. That's why they quickly developed it into the Patton series of tanks. Why do that if it was already good enough?
I don't expect historical accuracy from the game, it's just nice if things can be more or less as good/bad in different areas as they were in real life, so the game is more intuitive that way. That's one of the main things that interested me in CoH in the first place.