Andrew R Stewart 

I haven't had the chance to play with the Shimano 11 speed stuff but here's my take any ways. The behind the cassette spacers are to insure that the cassette (epically the smallest cog) fit onto the splines fully without bottoming out or not having the lockring completely engaged. Seems to me a pretty easy thing to figure out with the parts in hand. Whenever you swap out a cassette (even of the same cog count) you had better check the der's range as well as the SIS cable tension adjustment. To not do so invites over shifting into the spokes if all's just a little bit off. This is independent of any spacer needs. CHECK THE DER ADJUSTMENT EVERY TIME YOU SWAP WHEELS OR CASSETTES. Is that plain enough?

As to chain length- Again it's a quick thing to check. By trying out the current chain length with the new cassette. Slow goes it as you shift into the big/big combo just in case the chain is too short. A single tooth bigger a low cog usually is not enough to cause too short a chain, unless you already were pushing the chain short length limits.

Lastly a pet peeve of mine with this kind of question. Why do so many people think that it's a plug and play world? If it's written somewhere it must be the only correct thing to do? It seems to me that the ability to trouble shoot/inter pet/understand a system is now a days limited to what others say. The answers are in front of the person who is putting the parts together if only they would take the time to see the interplay they are creating. There now I feel better, Andy.