One Super high end cable or mid level set?

Let make the assumption that you're getting the last 10% of improvement for ten times the cost and the mid-level cable cost is $250. It would make far more sense to have 10 cables that are capable of 9 out of 10 compared to a single 10 out of 10 while the rest are only 1 or 2 out of ten.

Having said all of this, I think the last 20-50% is mostly placebo or at least not much beyond different rather than better.

Placebo or not, the scenario of buying one cost-no-object cable while having lower performance cables in the remaining system makes no sense because the limiting factor in the system's performance is the weakest link in the chain. So if you put Nordost Valhalla IC between a source and your preamp, then, the limiting factor will be the IC between your pre and power amp or if you are using an integrated amp, then the limiting factor will be the speaker cable between the integrated and the speakers. Because if the inferior cables are not resolving enough to carry through the "enhanced" signal that the Nordost Valahalla (carrying on with your example) is delivering, than you have wasted your money on the Valahalla. Much better to have balanced cables of equal performance across your system as you remove one source of imbalance in your system this way. Just my 2c worth.

IMHO I would buy all the cables I needed as opposed to spending all my budget on a single expensive cable.
There are some good sounding, reasonably priced cable combinations on the market today, and the synergy gained with all the cables replaced would outweigh a single cable in my opinion.

Don't limit yourself to Nordost..There are plenty of less costly and ( in my mind ) better value cables out there.....