The long term effect on the elevation of the area will be that it stays pretty much the same.
Explanation:
The isostatic uplift is a process where a land mass is gaining in height after the pressure on top of it is gone, most often because of melting of large masses of ice. The leveling is a process that occurs because of the erosion, which is destroying the physical features and gradually makes them flatter and lower.
If both of these things occur at the same place, the end result will be a relatively same elevation on the long term. The reason for that is that the elevation that is gained from one side will be getting lowered from the other side.
