As a life long skeptic…manufacturers carefully select components and wiring to optimize the performance of their devices…and advertise as such. Considering the margins involved and just how competitive their markets are, if they felt their device would perform appreciably better vs the competition with a certain power cord rather than the generic one they include, than they would include it and bake it into the price.
The high end is not about pinching pennies. Whether an amp is $22k or $23.5k would make little difference in sales volume if the $23.5k version clearly outperformed their competitors thanks to be paired with the perfect power cord. So, it would be my contention that if the power cord made the device sound better, it would be in the box. Just my opinion, of course.