Gearboxes have an overflow right? If you have to much it will overflow out the breather.
If you only have a little too much, when it gets real hot, it will expand and overflow until the level gets to the operating level.
It is potentialld damaging on an auto, but not too much of a worry. On a manual, you will lose power for sure as the shafts are not immersed in oil normally, they just touch it and it gets sprayed everywhere.