Well, see if AMD has a better design for their version of the 12VHPWR connector as their new generation of cards is using it too.Kinda a odd issue. A decent 8 pin(6+2) can push 300w all over 3 12v pins. Nvidia could have made a 12 pin of the same form factor that could do an excess of 600w. I'm also tired of seeing 4 6+2 going into these harness. Its unessisary, if you're willing to push a small custom connector close to its limit no reason to be so ridiculously cautious on the other end of the connector especially when all the 6+2 are normally tied to the same 12v rail.
If you are making a custom connector use copper tabs like server nodes. That can push over 1kw reliably and could be fed with just 2 high strand count heavy gauge wires. Then sell adapters over a foot or so letting you hide the 6+2 pin connector stupidness.