RE: GP Easy Sport balance.
Well guess what? I've built two of these guys! I love to start off the season flying the ES first before going on to my other models. The reason - less stress. And I get more confidence with our windy weather starting off EASY (Ha HA).
I don't know must about the design but I too had to add about 3 ounces to the tail in order to get it to balance at the given CG. I have an OS 46 installed and placed the battery and receiver as far forward as possible.