I found a prop thrust calculator. It is not perfect but at least i get some raw data to see if this idea has any merit. I think it is a good start.
If you want to play along here it is 
http://rcplanes.000webhostapp.com/calc_thrust.htm

I entered the numbers you provided in the calculator. 3B 13x7 and 7600RPM. I have left everything else default as a matter of fact i have left everything else default for the remainder of the exercise. What i did was to try to find equivalent setups that produce equal thrust and speed. After that i found something surprising here. Feel free to enter those number in the calculator.

13x7 3B 7600RPM    Results Thrust 118.2oz  Speed 60mi/hr Power 768W
13.73x8 3B 6600RPM  Results Thrust 118.3oz  Speed 60mi/hr Power 713.3W
14.86x8 2B 6600RPM Results Thrust 118.1oz  Speed 60mi/hr Power 711.4W
15.48x9 2B 5900RPM Results Thrust 118.2oz  Speed 60mi/hr Power 668.2W
16.25x10 2B 5300RPM Results Thrust 118.3oz  Speed 60mi/hr Power 652W

I think we are missing the boat here guise. We have not even started to explore the possibilities of Electric.  Electric motors are not restricting us to use a narrow RPM range as the IC motors do. We can do anything we want to do. The strange thing is when I was looking at the range of props that can be used in a motor it looked strange that fit them with unreal big props, I think this is why. After I played with these numbers what i found is that Russel can potentially save 15.1% power usage just by changing to another equivalent setup. Moreover lowering the RPM opens the door to go to 3S with a 650KV motor. That means cheaper batteries. I think another advantage is if you go active accelerometer the motor has a softer narrower range from upper and lower limit which makes the transition to the high or low RPM faster. The downside is the current as the system will have to handle more Amps on 3S and you need batteries with higher C rating. That is not bad at all.

I guess a preliminary conclusion 2B is most efficient for now (I didn't try 1B props because is ugly and requires thing i do not want to do). Another conclusion is that as we increase the pitch and lower the RPM we can use less power for sure and potentially cheaper batteries but we have to accommodate larger ESC. Lower RPM also means the gyroscopic precession is cut in half compared to the high RPM low pitch props. Also there is less load on a prop at 5300 compared to 10000+ so tey can still be light enough (sub 20g 15in dia prop) and have the prop stay together without braking. 

Traian