The couple of pounds in weight are are of little concern to an infantry soldier, if you consider the amount of kit used on patrol, ammo, water, more ammo, radio batteries, more ammo, 60mm mortars, 1 belt of 7.62 for the mag crew, first aid kit, food, signal flairs, map, grenades and more ammo, then those couple of pounds = a small percentage of total weight, i would take the R4 in any sandy/dusty environment at ranges less than 300M, due to reliability, i would take the AR platform for any longer range engagement.
Scope mount is not to bad, the scopes are attached to the top cover, you just remove the top cover and pop on a scope attached to a spare cover, only optic i have mounted in a night vision scope.
Attached image is of me with an R4, night scope attached, it would hold its zero for a full two weeks in the bush, even if it spent the daylight hours in my pack being bumped around. Only misfires i ever had with it were ammo related, nothing a yank on the charging handle couldn't take care of.
2nd image left side of the photo shows R4 with a rifle grenade in place