There are a few things that enter into how much powder you can burn. The kind and grade are at the top of the list. It is possible to tell by the velocity, when you quit gaining then you are blowing unburned powder out the barrel. The practical point of efficiency with most subs [which are a constant burning rate powder] is around 110 to 120 gr. You can get to the point if you push it far enough the the weight of the unburned powder adds to the pressure and recoil and reduces the velocity.
The only sub I know of that is progressive burning is Blackhorn and since the powder burns faster as the pressure increases you should stay with the advertised 120 gr limit unless you can do some pressure testing your self.