I'd be all up for some diminishing returns but the price premium is nowhere close to adequate. For the price of an overbuilt headphone amp you can buy a soldering iron and parts for three amps that are four times as overbuilt. And include a metrology-grade DAC in all of them.

And, yes, my headphone cable is oxygen-free copper. I simply chose the cheapest suitable cable I could find at Thomann, the stuff is so cheap they're throwing it in there for diminishing returns in sales.