Yep, I pretty much use 50/11 downhill or on very rare tail wind high speed flats. Not used much!
should be as both are shimano, not sure about the chain though
Im pretty sure the cogs in ultegra are
you only drop the 16 and jump on the granny.
The jump from 24 to 28 is a beautiful thing when buzzing up Nettle Hill!
I use a 13-27 Ultegra, which is also difficult to find, but made one up from two other cassettes. The sprockets can be interchanged, even if you need to use one or two sprockets from say a 105 or a Dura-Ace. All the sprockets from a given speed range, will interchange. eg: a 16T from a 105 will work with an Ultegra cassette, or Dur-Ace, providing they are all from 9 speed or all from 10 speed.