It doesnt make any sense that 6PIN cant deliver more than 75W.
Suddenly 2 extra cables give 75W more?
Also it seems the 2 extra cables are ground only, check the image:

For me it rather makes sense that some hardware on 480 is routing too much power from PCI-E, while it should just get much more from the 6PIN.
Honestly I have never mined with powered risers... I am using MSI boards with EPS12V (8PIN), maybe that saves my boards

The usual molex ones with basic resistor look so crappy to me that I am afraid to use them

The usb ones look decent, but you say they are not that good?
Any good boards besides Asrock H81/61 Pro Btc that you can recommend? (to handle extra PCI-E power)