Crist,
in your experiment, what is your goal, to maintain the airspeed, or to get the airplane to fly correctly at the higher weight. Your experiment has some interest but, until you establish exactly what prameters you are controlling and reaching for, it preobably is not conclusive, at least to me. Now another thought, in your test flights, I totally understand why you only did short flights, however, I think one factor that throws your results off, you didnt do any of the overheads, the vertical eight, the hourglass, the hori 8, and these are the real tellers as for power.
I agree that there is a lot more to be considered than simply weight. the ULTIMATE thing, is how the airplane responds, how it flies on the handle, and more important, how does it score. In my opinion, you really cant use partial flights as an indicator of what is really happening. It is a good starting point to explore but until you add the weight, then fly full patterns back to back with the light weight and the heavy weight, and the airplane trimmed for the weight, I dont think that really much can be said. If the airplane is not trimmed for the weight, then you are using more control deflection than you need to and thats drag and thats more power.
I was pretty surprised with my electric P-40 that weighs over 57 oz on 570 squares of wing. It flies extremely well despite the weight. I really dont want to add weight to this airplane for obviouse reasons, and leaving weight off isnt really an issue.
another thing I think to keep in mind is that if you are doing this experiment with purpose to use the info to set up a new airplane, keep in mind the Primary force doesn have flaps and the addition of flaps will potentially increase the power drain in manuevers from drag.
To make a long story short, I like your idea, to quantify the affect of weight on power drain, I think perhaps though a few more valid tests, full patterns, would really help to solidify what you are trying to show.
keep up the good work!