I've also setup Ultegra Di2 with an 11-32 cassette without an issue. It depends on the precise geometry of your dropout and the chain length, but with a 30-tooth cog it will probably work fine on most bikes, and on some you'll get away with a 32. Also remember that you can switch the inner ring on a compact for a 33-tooth (not available from Shimano, but is available from Stronglight or TA).
Thanks for the replies. I have one of these:http://www.bmc-racing.com/int-en/bikes/2012/road/model/roadracer/sl01/sl01_ultegra_di2.html
I noticed that the '13 plain Ultegra model has the 12-30t cassette:http://www.bmc-racing.com/int-en/bikes/2013/road/model/roadracer/sl01/ultegra_.html
I don't believe the frames have changed from the '12 to '13 model. Presumably that will work then?