I still think it might work ok with a 3 cell battery and a 7" three blade prop. A 3 blade 7" prop is about equivalent to a 2 blade 8", which is the recommended prop for three cells and so should pull close to 13A.. lets call it 11A.
11A x 10.5V = 115W per motor.. thats 460W total. Thrust should be around 450-500g per motor, so about 1800 - 2000g range total.
if an 8" prop could be shoe-horned in then a 8" fine pitch 3 blade should give even better figures..... A fine pitch prop would be much more efficient for a model that's designed to hover so the 3 blade GWS slow fly props are worth looking at.