I doubt that the Ostheer mortar is allot better but that is not an issue here. If the mortar replaces the Pack Howitzer in Captain and it need a buff it should receive one.
The Ost mortar is a lot better. The USF mortar ends up being an overpriced smoke delivery unit. Maybe you're thinking of the Soviet mortar, which is only a tiny bit worse.
I wouldn't mind making the Pack howie more like an artillery piece, if they would at least boost the USF mortar to match the Soviet one. The Ost mortar would be OP if used against Ost because of having to keep Grens stationary.