I think I should correct you that your assumption is wrong. I run the underslung shotgun quite a bit and I have done testing on this issue. The spread on the underslung shotgun is affected by steady aim. However, the built in laster sight on the ARX doesn't affect it at all. The shotgun's spread is identical reguardless of the hipfire spread of it's parent weapon. Try this yourself. Make the following class.
Primary Weapon: ARX
Secondary weapon: Ameli
Perks: Overkill and fully loaded.
Load up in an offline or private game with the class and shoot the wall with the underslung shotguns. You will find that the spread on both weapons is identical despite the ARX's built in laser sight and the wider default hipfire on the LMG.
The underslung shotgun is still a reliable secondary weapon reguardless of what you bolt it to. Also, you can tighten your spread slightly by ADS with it. It is also affected by the perks, sleight of hand, steady aim, quickdraw. Scanvger and fully loaded will both supply additional ammo for it. It's a nice attachment if you don't like pistols for close range and it suits the ARX;s ideal combat range. It just doesn;t have a tighter spread from the built in laser sight.