Shimano redesigned the Ultegra rear derailleur this year to optimize performance and decrease weight yet again. The beauty is that all the performance enhancements of Dura Ace come out in this new Ultegra piece. Well, everything short of the super pricey carbon fiber cage. The wider outer link reduces shifting deflection, while the redesigned cage is optimized up to the new 28 tooth cog available in the cassette. And of course, it's lighter too
Wide link design to reduce shifting deflection
SS rear derailleur gear ratio is equivalent to triple without changing to GS rear derailleur.