The other point is that most high end power supplies are single rail. That means stealing power from the 12V CPU power connector with some wacky chinese adapter to put it into a PCI-E hookup is pointless. All the power is coming from one 12V rail anyway.
I would simply buy the right power supply instead.
I'm pretty sure everyone here understands the EPS and 8-pin PCIe are different. That's why the OP started this discussion by asking about an adapter.
If you already have the PSU, and it's capable of sourcing the required power, but lacks the required number of 6-pins, it makes more sense to use a $5 adapter than to buy a new $100 PSU. Sure you could splitter your existing wires into more 6-pins, but that puts excessive load on cabling while leaving other several-hundred-watt-rated 12V wires unused. And still requires buying splitters - which is to say, a different type of adapter. His question, and intended purpose, are fully legitimate.
Additionally, not all adapters are "wacky" or "chinese".