If someone puts a huge effort into making the best amp they can, and they don't spend a cent on making the power cable any better, there's your proof that the amp designer can't extract any more performance from it due to the cable - otherwise it would have had a different cable. And it seems close to every amp designer in the world agrees on it too....
Perhaps think about it this way...in many industries with high end products, there is SOME guy who specializes in some part that other high-end manufacturers just use 'basic high-end parts' for which are not the ultimate, ultimate, ULTIMATE version. You can buy an expensive sports car...150K+...and some custom seat guy will give you a more expensive custom racing seat. Some other guy will give you custom run-flat tires, special rims, add a whole new stereo system worth 75K, special pistons, etc....bullet proof sides and windows. you can ALWAYS MOD with 'upgraded parts'.
That's the whole point of upgrading/mods, etc in super-high-end hobbies...people always want to push the performance envelope by going to specialists in every little part to really get the main product even better. and i suspect car manufacturers would probably acknowledge that this market is legitimate...but not their own core business which is perfectly understandable.