I had heard the DA 7900 short cage rear derailleur would shift a 30t rear cassette without issue. Good to know. I am going to run a 11-28t cassette with mine. Ultegra cassettes are cheap and come in a no. of different sizes and to me a big allure of Ultegra 6700 or DA 7900.
DA 7900 has taken some criticism as it isn't perfect...but many of the rear shifting issues as with all groupsets is due to poor setup which is common if not prevalent in the industry. Lots of botched cable jobs out there. Specialized is known for this on their internally routed cable bikes, one of which I own. Spesh released a new routing spec called the California Cross that makes cable housing radii a bit more free flowing.
DA 9000 sounds to be the cat's @$$ and preliminary reports are its the best mechanical groupset in the industry. Of course it will cost a lot...require a new wheelset or redishing for the longer 11s free hub and cassette but Shimano has pulled out the stops with revising cable pull for both front and rear derailleurs, further tweaking of hood shape in the direction of Di2 and much shorter shift throws.
For a more budget conscious rider, DA 7900 should be a good sweet spot...will see.