I’m not familiar with the MI State laws (I live in GA). Is a SBR an option? If so, I’d go that route. This means one of your lower receivers is registered as a Short Barreled Rifle, allowing you to attach an upper with a barrel shorter than 16”.
If the SBR route is not an option, personally I would probably sell the upper to someone with a machinegun or an SBR in another state.
Why would I do this? In a word, velocity. If you really wanted to keep the upper I’d suggest mounting the 5.5” (like the one
here at Bushmaster) and blind pin it. This will bring your barrel to the legal length.
Why would I choose to take a small hit to the wallet by selling that upper/barrel and buying another? You loose a lot of velocity with a 11.5” barrel and a 5.5” flash suppressor. You have the same length as a 16” barrel, but with a loss of velocity. You already know the benefit of a shorter barrel when moving in and out of vehicles and buildings. You are willing to trade velocity for that gain in mobility/size. Sticking a 5.5” FS (or fake can) on the barrel looses the size benefit and also give you a potential performance hit with the loss in velocity.
Hope this helps.
edit fer speelin'