Go to Naim's own forum and they will tell you a Flatcap XS (the Flatcap 2 and 2X are discontinued now) is essential and your life will be unimaginably better with it.
Rely on my advice and you will be reading someone who was so knocked out by the CD5X and Nait XS combination that...
(a) he never wants to hear it again in case of - financially terminal - temptation setting in.
(b) was so impressed he cannot remember if they were used with a Flatcap or not!
(c) Hates extra boxes almost as much as he loves his current system.
So, basically, as a Naim system owner, I am going to be rubbish at giving Naim advice
Your best hope for advice is to wait for one of our two 'tame' Naim dealers to come along and make some sense. (Dave @ Frank Harvey Hifi and Rick @ Musicraft.)
It would be better value if you had another Flatcap XS compatible device (Nait XS for instance) because it has two outputs.
In the case of your CD5X, the Flatcap will only be powering the analogue output stage and with the Nait XS it will power the pre-amp stage.
A new Flatcap XS costs £675 Can you identify any areas (speakers?) where the additional money will give you more improvement?
Probably best to get a loan unit from your Naim dealer for a home demo.