I have the same setup with my GP Extra 300S and it flies fine and hovers with the 9x3.8 prop. I haven't tried it, but I also bought the 8x3.8 to try. I also didn't try the 10x3.5 that came with the plane. I was told that the brushleess motors don't have the torque compared to brushed, but the RPM's are greater and therefore a smaller diameter prop is necessary.