dave
Plywood King
I tried a boutique power cord (not Naim). No difference. Rien. Nada. A mate did something similar with similar results and was sufficiently angry at being duped as he saw it that he sent off a vitriolic email to the supplier. If the Naim one makes a difference to your Naim system, and it's worth the £400 to you, good. On all the equipment I have owned a decent quality IEC lead is good enough and boutique cables don't imnprove on things.
Oddly, the very few boutique power cords I've played with using assorted round and flat earth gear always sounded worse than the stock appliance cord that came with the gear. The differences weren't huge but enough to make any decision easy. Like most aftermarket add-ons, I think the companies just screw with design parameters enough to make a change in the sound hoping the end user will confuse a change with an improvement.
One exception was buying an uprated version of the stock Belden power cord that came with my Linn amps. I was careful to maintain the exact cable and plugs' catalog numbers with the only difference being a current rating increase with a change from 14 gauge to 12 gauge wire. The improvement in sound was small but audible and worth the $15 US I spent for each. However, in the case of Naim, nothing I've tried at any price beats the Volex or Mayor appliance cords that came free with my olive rig when new. TBH, I'd rather invest any serious money in better gear considering what you get in return with any cable whether power or interconnect.
regards,
dave