Gatt, sustained turn rates and "E retention" or drag are intimately related. Of the guys posting on this thread, the only one who has even a remote idea of how to calculate such things is Wells, and he seems satisfied with the new drag model in level flight and coordinated turns.
I think the issue now is drag due to large yaw angles. There is either too little drag or too much lift from the fuselage, or some combination of the two conditions.