I went 11 speed DuraAce Di2 a few weeks ago on an existing bike, but I did it for Di2 rather than 11 speeds. I can't say I've even noticed the extra gear (my 10speed DA cassette was 11-28, and my 11 speed DA cassette is also 11-28), but the Di2 sure is noticable (and, totally unnecessary and awesome at the same time).
As for wheel compatibility, all my other bikes are still 10 speed but my goto wheelset has a powertap. Powertap wheels have a freehub that pops off in a few seconds so I have a 10 speed freehub/cassette and an 11 speed freehub/cassette. When I swap my wheel to another bike I just grab the assembly I need and it goes right on, no tools needed and like I said, it only adds maybe 15 seconds to the swap time.