VM and efficiency are two words you don't use in the same sentence unless speaking in negative terms.
If you can get an expansion chamber on there instead of the 10oz along with a reg you will be in better shape.... other wise you will have your gun valved to run both gas (from the 20) and liquid (from the 10).
The math used to be about 400 shots from a 7 oz tank... on a warm day.
If you really want a good running vm...get a VTS (cough) I have a vm with vts for sale(cough) put a reg in the front asa and a 20 in the vts... you'll be running gas with no liquid and it will be entering the gun at a constant pressure. Then you can get some good efficiancy.