Has anyone been able to pull off a realistic Flat Spin in G2 with one of the big 3D Birds ?

Try the Edge 540 Freestyle (SD), or the Extra by the same designer. They'll do a nose down flat spin. You probably already know RF doesn't have the low speed physics to perform this manuever correctly. These planes will get you sorta close to realistic on a lot of 3D manuevers, but they can get a little unrealistic if you let them.

My H9 funtana For G2 will do great flat spins. dropping straight down and the plane is almost perfectly flat. I also make all the (SD) planes. the funtana should be on the swap page soon.