First off, I bought the CD5x/Flatcap2x together, so never heard the CD5x without it.
CD5i and CD5x seem to have the same Naim sound characteristics. Very dynamic, really gets your toes a tappin'. I would say that the CD5x/Flatcap2x combo probably improves on the basic sound of the CD5i by 30-40% (if I can put it into numbers).
I only had the CD5i for about a month, but as I said above, was really impressed. I compared it to a Rega Saturn and McIntosh MCD201. It bested them both, IMO.