The tanks you listed are tanks, and all heavy tanks to be precise. The strumtiger is an artillery piece, they both have diffrent roles and thus you cannot compare the two.
At the moment the Sturmtiger is an assault gun. But u need munition to fire a single rocket grenade. Effective against infantry and light vehicles.
It will probably be quite comparable to the KV-2, except with no turret but (maybe) better armor and health.
Please keep in mind that CoH is a computer game and we have to balance it. So historical data of the weapon are fine and interesting but not important for CoH.And keep also in mind that the Sturmtiger is a reward unit. When it is too strong we can delete it.
I can't wait till they add british to CoH:O; the extreme forces leveled by the new imbalances will create a black hole around the servers that destroys half of Canada.