How important is powercord synergy?

If you can hear the difference, then it's important. I've tried about 20 or so PC's and some made a big difference, some didn't. System matching/synergy has to go hand in hand with what you want to hear.

Also, a cheap power strip/surge protector may do more harm than good. I'd suggest plugging directly into the wall to determine if there's any difference. Additionally, if you don't have dedicated lines and/or upgraded outlets, you might want to look into these.

Lastly, when you buy a PC, you may be surprised to find some of the lesser priced cords do a very good job, especially in the DIY area. I have two in my system & am very happy with them.

Powercord is important depending on length and current it needs to drow from the wall socket. If you intend to use a long powercord you should take a thicker gauge

If you take care of what's inside the wall and wall sockets you may find little-to no differences with properly selected powercords.

A power conditioner is great if you have enough voltage i.e. 120..122V and assume a loss of voltage in conditioner by a couple of volts.
Amps are OK if input voltage is a-bit higher but "hate" lack of voltage and bring an unpleasant distortions.
So I'd rather think of PC later than sooner before certain things are taken care prior and maybe you won't need it after all.

Dedicated lines will make bigger difference than any power cord upgrade. Improper power cord can damage your amp, this is why you will always get a cord that can pass enough current for your amp to last. Don't use cheap power strip, this could damage, or at least degrade your amp. If you can hear any difference a power cord change might make then go for it, It would be last on my list of upgrades but as always in audio, keep an open wallet. (I mean mind).