Yeah, it's basically gonna be the same level of physical change as the drum mod is - though you would become eligible for the Increase Cylinder mod.
That said, it seems to me that you may as well just go with the Additional Clip mod to add a second internal magazine, and the Ammo Skip System with one additional slot to represent being able to grab from any point in the mag.
In either case, I think you're looking at a change to Concealability (still something of a factor for shotguns), as well as either level 1 or level 2 Custom Look being applied - though likely without needing slots, as the benefit is coming from the appearance shift cause by the above mods.