I’ve never done a direct comparison between the two alternatives you mention. To the best of my knowledge the circuitry in the external Naim phono stage is identical to that on the boards I’m selling. I’m not familiar with the NAC82 but I believe that some Naim amplifiers have provision for an external power supply for the internal boards. If this is so and you are contemplating that route you need to be aware that the wiring of the SPX is different to that of Naim’s external supplies.
These boards draw minimal current from their power supply rails and there will only be a slight advantage, if any, from an external supply.
Fitting these boards internally will reduce your box count and reduce the expenditure you make.