First of all, generally speaking, in order to fly a 15-16 lb airplane, you would go to 10 cells minimun and better is 12 cells. I would consider a 6 cell pack for each motor and might consider going to more. The first thing I need from you is some more info.

I need the maximum prop size that you can use on this aircraft. I need wingspan, and wing area. What kind of thrust would you like to have? I would go for about 1.2 or so to 1, probably with a prop speed of 70 plus MPH or so. Does this souln right for you? I would be glad to look at some possibilities for you. I will wait for the additional info.