That flair in the back of the mill is designed to increase the speed of the air flow and hence reduce the pressure behind it. That should give more power, but now you have in effect increased the capture area. That should be taken into account. Other effects also come into play. The tip vortices are eliminated as, Ron mentioned, helping efficiency. But another effect, probably most pronounced in small mills, that would be eliminated would be radial flow out of the mill caused by the pressure difference between the air flowing through the mill and the air flowing around it. That flow would be forced to zero. More air would flow through the mill causing operation to move higher on the efficiency curve. If you're wondering what curve I'm talking about, you will find it in my diary titled "The Betz Limit - Again".
So, should you try putting a duct around your mill? Sure, what would it hurt? It's not hard to laminate wood and fasten it to the blades. If you need more support add some round struts. Keep it simple. A simple band. You need not extend it in front of the blades. Have fun!