If you are interested in testing for energy efficiency, you really need to test with the payload. Changing other variables such as angle of attack is only possible at different airspeeds, otherwise it will result in climbs and additional energy loss. There are really only 2 things to set - altitude and airspeed. Other variables have to be tested. Fluid dynamics are inherently nonlinear, and it’s usually easier to just test instead of model when it comes to efficiency.
If you want to know some numbers about twin-engine airplanes, you can see my results here: Benchmark Test of the Twin Engine Mapping Planes