Base form, I'd say lifting and tossing a car is near the height of his strength. Fused, he'll be lifting that same car with one arm. Surya himself could pick up a subway car and wave it around like a toy.

Now, each of their power varies with their level of anger at that point, so there would be moderate shonen-esque divergences from the stats I just listed in certain scenarios.

Forgive me if this sounds powergame-y but I'm hesitant to give specifics about Monshiro's strength because I want to ensure that he has the most physically powerful contract. Sheer strength is essentially all he has, and if he ends up outclassed in that regard, his contract would seem pretty pitiful.