I had an ND5XS but never tried an external PSU in my system. I did audition one with and without XPS and the difference was subtle to say the least. Its allegedly the same DAC unit as the NDX but with an 'inferior' PSU. So theoretically, adding the external PSU should get you to the same point as an NDX with external PSU, although this seems to be a moot point with Naim!
I really enjoyed the ND5, very well built and sounded lovely, but I bought a Chord 2Qute when they came out and ran them side by side for a while and preferred the Chord with the music I predominantly listen to. I now use a CuBox micro computer as a renderer on my network to stream using Roon or LMS as the control apps, both of which I prefer to the Naim app. I did try feeding the Chord from both ND5 and CuBox and they sounded the same so it seemed daft to keep the ND5 just to stream so decided to sell it on.
TBH, as Keith suggests, by far the biggest upgrade I've made in my setup has been achieved through improving my room acoustics, in my case using a DSP to correctly integrate my Subs and apply a little EQ to smooth out the bass response. The other major 'upgrade' has been a Dirac Live digital room correction processor for when I'm listening at my main listening position. Simply fabulous and way beyond the improvement that any change in DAC or PSU could possibly deliver IMHO. A lot of measuring and learning involved, but well worth the effort.