Personally, I couldn't do better than 3 inches at 100 yards with Marlin's factory sights even if the gun were chambered in 6mm Benchrest. Some folk's can do wonders with those sights but they are few and far between.
You may be much better than me but I think mechanically your limited to around 2 inches at best, tack on another inch or two spread becuase of the sights and I'd leave happy with nice round groups of 3 1/2 inches or so. Which is plent good for 90% of Deer shots in my area. Good Luck!