I have a 2.5x8 Leupold with the heavy duplex on my Chapuis 9,3 double. At 2.5x I have plenty field of view for close range shots and I like having 8x for certain conditions. For example when hunting animals that tend to travel in groups the higher magnification allows you to pick out the correct animal and keep the crosshairs on him until you have a clear shot. This is a must on turkeys as they are allways moving around and you can "beard shop" and be ready to shoot when the big one is clear. The 2.5x8 is also better in dim light than a straight tube scope.
I am also going to mount a 2.5-10x50 Ziess with illuminated reticle. I will use this scope for stand sitting, hunting over bait or at night. That is one advantage of having QD mounts, you can have 2 scopes sighted in and ready to go.
Most of the game I have killed with this rifle has been with iron sights, but for some hunts a scope is necessary.
My 9,3 double with a scope still handles like a well fitting 410 shotgun. The scope is mounted low to the bbls and "mounts" perfectly, much better than any other rifle I have handled.

A scope on a 9,3x74R double moves it into the "All Round" medium rifle catagory. Most of us just plain hit game better with a scope at ranges past 75 yards.