The Dura Ace outer chainrings are far stiffer than the Ultegra rings, which causes the shifts from middle to big to be much smoother (the chain moves rather than the outer ring being deflected). We're using a 7800 Dura Ace outer ring on the tandem, and it works better than any other we've had on there (previously had an FSA and an Ultegra). There are no Dura Ace middle or inner rings with a normal BCD, so the outer ring is the only place you can use one; the new DA rings (7900) have special shaping to match the cranks, so the 7800 outer ring is the ultimate IMO.