I'd say you have too much prop (diameter). My Saito 1.20 turns an APC 15X8 at about 10K. I have not heard of anyone running a Graupner 15X8 on a 1.20, the equivelent to the APC 15X8 is a 14X7 three blade. My plane was very fast, and needed the controls to be tamed down a bit. It will be slower if I run it with the three blade prop though. I set my CG towards the front with the gear down, and about the middle with them up. I actually had to move the battery back some to achieve the right CG. I would not recommend taking off with flaps, especially on a maiden flight. It seems to be a good way to crash though. Flaps help a bit on landing, but you shouldn't need them on take off.

Put a different prop on it, leave the flaps alone until landing, make sure the CG is in the front of the range with gear down, and you should find it to be a good flying plane. Also, make sure you have enough throw to get it trimmed out. An untrimmed plane can feel like a handful too. More weight is never a good thing. Only add weight to balance, and even then only as a last resort. I have never heard of a case where a plane flew bad just because it was too light (other than a windy day).