I also have a dedicated 20A line for my X350.5. According to the manual and most of the opinion on discussion boards, the stock cords should be good enough and any high price powercords would only give you a marginal improvement. I've not used the stock cord, so I can't comment on that. When I have some free time, I will go back and try the stock cord.
I was just having a discussion with a buddy of mine last Friday night. He doesn't have a dedicated line and he tries new things more often than I do. He told me that in his system, the after-market powercord made a big difference compared to the stock cord with his X350.5. So this may be a case of relativity. Both of us use BMI Orca limited Mk IIs. He had used the Shunyata Python Alpha and ended selling them and bought bunch of BMI cords instead because the price is a lot cheaper and performance is close.
If you've been surfing Audiogon long enough, you may be familiar with BMI. They used to post tons of ads here selling powercords. I think that's how they got started. But they had since focus on the Asian market exclusively and the prices went up 5 fold (at least their list price did). So the only way to get them now is through Audiogon or eBay.
FrankC