they lymans are great guns, .54 is a great caliber. I have a hawken 54 that I've killed some deer and hogs with. love it.
you are going to find that traditional muzzleloader hunting is much like bow hunting....its a skill that you will have to develop and maintain. its not going to be like a centerfire or an inline where you just sight them in once and stick them back in the closet until you're ready to go.
a traditional lyman like that will definitely take a deer at 100-150yds....bullet will definitely drop some over that distance....how much will depend on where you zero the sight. but you're going to have to do some endless practicing to be that accomplished with that iron sight. its doable though. when I hunt with mine I just won't shoot that far. I keep my shots within 100yds. because of my capability...not the gun.