The planes are quite different. The Sig SE has the wing in the fuselage center and the GP Extra has the wing at the bottom of the fuselage. This causes a considerable amount of roll-pitch coupling with rudder application in the low wing plane. Also the larger fuselage causes more drag. The roll-pitch can be mixed out with a computer radio. It seems as if lack of power may be a problem. You shouldn't need full rudder throw to maintian knife edge flight as it will just slow the plane. Center of gravity location can also effect the knife edge flight. Knife edge needs side area and speed to maintian flight as the fuselage is now the wing (flying surface).

EXCAP232