With the upper wing at a higher angle than the lower one, I'm surprised you are satisfied with the flight characteristics. My experience with many large bipes (2 Phaeton 90's, three Phaeton 40's, seven 60 sized Aeromasters, one Giant Airomaster and about 5 other 1/4 scale) all flew best when the upper wing was about 1.5 degrees less angle of attack than the lower wing was. Especially on the Aeromasters, when the upper wing approached the same angle as the lower wing, the plane tended to porpoise a great deal when you tried for level smooth flight. Of course CG is also very important as is thrust line when you tune for best flying characteristics. Now, if you want maximum endurance and always fly at one speed, you can trim it out with the upper wing at a positive angle relative to the lower one but you pay a big price in maneuverability and constant trim at all speeds.