Modding NAD system: adding IEC & power cord ?

I agree w/Wainwright. It's highly doubtful that modding NAD's power cord is going to result in better sound. Just my opinion after having used NAD preamps for many years.

I made changes you refer to on an Onkyo DVD player, and it made a huge difference.

Suggest you replace the pre-out/amp in connectors on your amp with after market rca jumpers, if you have not already done so.
You could try a regular interconnect cable first, if you're skeptical about the benefits.
You will probably get a bigger benefit from this than trying to replace the power cord.

I would highly recommend you replace the stock power cord on your amp as well as your CDP. Hugh improvement. I just replaced the stock 18ga. cord on my Rotel dual-mono RB-960BX amp w/ a 14/3 Belden cord w/ a Marinco AC plug. I decided to not to put in an IEC chassis plug, but hard wired using wire nuts. Night and day difference. Used a "floating" ground wiring method on the AC plug end. Did the same w/my vintage Rotel CDP. Used the same Belden wire but I decided to use an IEC chassis plug. (Around $40 for a 6' PC w/Marinco IEC and AC plug.) Wonderful sonic improvement. Take Five Audio in Canada has all the supplies you would need. You'll be amazed at the improvement.