The new Ultegra R8050 Rear Derailleur will do 30t (like the new Dura Ace) on a short cage and with the medium cage is supported to 34t.
I've heard many people have successfully used a couple more teeth, but you do need to make sure the RD clears the largest sprocket and that the chain is long enough, but not so long that it's loose in little-little.
Interestingly the new Dura Ace R91x0 Di2 refuses to let you shift into small ring with the smallest 2 sprockets (34-12,11) to avoid low chain tension, instead it changes down at the back for you (or ignores your request to shift up but Synchro shift avoids this by changing up for you). This suggests that Shimano are concerned about chain slack with a 16T difference on the front.
Cross chaining is not really so much an issue with Di2 as the front derailleur auto-trims, so there should be no rubbing on the front cage, but I guess it is a little less efficient.
I got a new battery for my old bike (and the bluetooth thingy) to setup the Synchro Shift (just program it in e-tube) and if I try to shift into the biggest sprocket in the rear (from 28t to 32t) while in the big ring (50t) it first automatically drops the front to the small ring (34t), then actually shifts the rear up two gears to 22t to compensate, so it never goes 50-32.
The Synchro shift up to the big ring is lovely and hardly noticeable, but the down is a bit of a surprise as it suddenly gets lots easier, before quickly moving the back to compensate. Hence I only use the synchro shift down to stop trying to change down and finding no more gears and to avoid big-big!
Incidentally, the bluetooth dongle (EW-WU111) displays the gears on my Garmin (along with shift mode and battery level) so I've no excuse for not knowing what gear i'm in!
Geoff
