Sorry, made a mistake. I was so fixated on getting the max. drive efficiency at the airframe's best climb speed that I overlooked that the motors were underloaded. One cell more (6s1p 12 Ah for each motor) brings them into their range of best efficiency. Overall drive efficiency is now 50%, initial climb rate is 3.9 m/s (760 ft/min) at 12 m/s (27 mph) airspeed, and maximum height is 9800 m (32k ft) at 13.5 m/s (30 mph) airspeed. Two 6s1p 10 Ah batteries would still give 8100 m (26.5k ft) maximum height and two 6s1p 8 Ah would give 6500 m (21.3k ft).

Sounds fine especially regarding the structural benefits of having the heavy drives outboard on the wings. Counter-rotating props would be fine as well but I think there are no left-turning props. Could check now the same airframe with a bit higher-kv motors and/or a different airframe with faster (less cambered) airfoil and less aspect ratio. Would give small differences, though. What would be really needed is different prop pitch. The props linked to in post #113 look interesting and the aeronaut props as well but I have no data about them.