Pwr Cord Upgrade for PS Audio Pwr Plant Premiere?

Yes it matters how 'dirty' the power is to begin with.
All regenerators are making a new sinewave signal as any amplifier could. Just more of it. The output sine wave is NOT perfect. It is better than the incoming, but not perfect. The residual 'dirt' is about a tenth of the incoming 'dirt' So if the incoming is cleaner. the output will also be cleaner.
Mine is usually like 5% distortion in, and 0.5% out.
I use a Pangea AC-9 on the iinput of my Ps Audio PPP, and on my PS Audio P600. And also on my Furman REF20i (the Furman HAD a 20 amp IEC, which I swapped for a 15 amp one from Furutech which has 10gauge wire capacity)

So: the PS Audio power regenerator do benefit from aftermarket AC cords in, and out. IMO.
Others opinions may vary.

I used to have the PPP and tried various aftermarket power cords to no avail. I tried one of PS's Statement cords too. I ended up in selling mine because I could not get good sonic performance from my system (my speaker's sound was rolled off in the upper mids and high frequencies). I would suggest that if you plan to keep it, just use the stock power cord. From listening with and without these conditioner/ regeneration devices over the past years, I'm beginning to feel that if you have upgraded aftermarket outlets (maybe dedicated lines), you may not need these devices to acheive good sound.

Your best bet is to feed your power regenerator with a power regenerator! ;)