I guess it's all up to what he wants, a semi scale plane, or a fun flyer, I know I had a magnum .40 abc in mine, it flew just fine, many people try to over-power a plane like it's a 3D monster, but in reality, the GP kits aren't 3D machines, so maybe just sticking to the 40-46 range will teach energy management and flying on the wing. I have the GP .40 kit sitting at my ankles waiting to be built, this will be my second kit, the first suffered from old age and abuse, but I loved the way it floated in for landings when the CG was pulled all the way back to be very neutral in response. In fact, one day, I wanted to see the slowest I could fly it, so I chopped the throttle and feed in the elevator, the next thing I know it's coming down in a harrier move and I'm working the alerions to keep it level. It was going about 2 feet forward for every 10 feet in lost altitude, but the nose stayed level and it never tip stalled, even though I was working the alerions to maintain level. I think it will be the next on the board to be built after I'm done refurbishing my Pica Jungmiester.