I have resisted scoping any of my Marlin leverguns but was always curious how much better they would shoot if scoped so yesterday I mounted a Leupold 1-4x shotgun/muzzleloader scope on my Mountie and tested three brands of ammo.
At 25 yards:
Federal 550 bulk ammo: groups all could be covered with a quarter and some with a nickle.
Aguila SE subsonic: all groups nickle and two with a dime
RWS Target: All groups nickle or smaller, one was 3 in a hole with two not quite touching.
I wasn't able to shoot at the longer ranges due to hunter sight in.
Before I left I took the scope off and put the ol' factory sights back on and shot a few groups. While the scoped groups were superior, the open sight groups were close to as good. I would say within a 1/4" and probably closer to 1/8" on most. The difference was most evident in the RWS ammo and least so with the Federal.
I also scoped my Guide Gun to experiement at longer ranges. While I didn't shoot my best ever groups with it at 100 yards it was slightly more consistent. I think with practice I could get five in <1" with a scope. I believe that in low light conditions the scope would be superior but I prefer the handiness of an unscoped rifle in the field. Luckily, my eyes are still good enough for the factory open sights and I will continue to hunt with them in this configuration as long as I am able.