That seems very light. 80 grains of a 2f powder, granted would have all the power you need for deer hunting, but I personally feel they are able to take much more then that. Most of the .50 calibers are rated to 110 grains of 2f and 100 grains of the 3f powders. BUT I will tell you to stick to the manufacturer's recommendations.

The legal considerations might have something to do with their powder recommendations. It might also have something to do with the shape of the stock and the fact it is a full stock. So they might be suggesting this lower charge to reduce any stress that might possibly be produced on the stock. A friend of mine shoots a Tradition's Kentucky .50 caliber and he always shoots 90 grains of Pyrodex RS and a patched roundball. Whether that helps you or not... Good luck with your rifle.