POTS 8 is the name of the last upgrade of the board and potentiometers of the volume and balance. controls.
More info in FAQs
Here is one I changed earlier actually POTS 5 from a NAC82 but NAC 52 are the same:-

They are actual works of art I reckon, includes what must have been early use of brass weights on the boards.
I can't comment on audio improvements as I had several things done at the same time. Others have commented that if you are at POTS5 or 6 then it is worthwhile from 7 then maybe not so much.
E-Mail NAIM they will give you the latest price.