Good point! Yes it would be much simpler. Nevertheless a single partner does not integrate dual 6-pins design into the cards anymore (even though GPU architecture is more power-efficient than ever before) – in a way they are suggesting that 6-pins power supply board design is history, and care no more? Yet the Polaris architecture maker AMD implemented 1x6-pin connecter in RX 480 reference design with typical board power specification of 150W. Its power requirement bill seems perfectly fit the power budget of Mac Pro 2000-2012 or even older ones. Is it the-match-made-in-heaven for cMP fans?
You could possibly get away with 2xmini 6-pins to 8-pins or Y-splitters setup, which is quite feasible albeit at your own expense. (Direct mini 6-pins to 8-pins is a lot more at risk!) The discussion found in online forums (MacRumors) and expert reports (macvidcards) is reasonably convincing. But it is only natural to think that the threshold exists and depends on the card's power specs and choice of cable, as well as the factors mainly concerned with degree of overdrive and wiring configuration. One of the MacRumors members from Can the 6-pin gpu poewr cable in the Mac Pro be used on a 8 pin gpu like a 770? at #15 post explained it the best:
"It is clear to me that using a video card with either a 6 pin/8 pin combo or a dual 8 pin combo in a mac pro without an auxiliary power supply is exceeding the power budget of the computer and therefore posing some risk of intermittent behavior or possible component damage.
Does this work acceptably for some people? Sure. there are variations in tolerances and headroom factors that come into play. But there are no guarantees that this will work acceptably for any individual computer. Caveat Emptor!
There is absolutely NO reason for a graphics card manufacturer to put an 8 pin connector on a card UNLESS it requires more than 75 watts at the connector. The 8 pin connector is more expensive and takes more board real estate. The 8 pin cable is backwards compatible with the 6 pin connector, so there is no concern with the wiring harness of the computer. If you only need 75 watts at the connector, you can plug in either a 6 pin cable or an 8 pin cable.
It is technically possible to design a card that can sense whether a 6 pin or 8 pin cable is plugged into an 8 pin socket, that is the purpose of the second sense line. It is therefore possible to have the card modify its power profile depending on the cable plugged into it. This however is likely to increase the cost of the card, and therefore unlikely for consumer cards. It is cheaper for the manufacturer to put on the 8 pin connector if they need more than 75 watts at the connection and publish the overall power requirements of the card. If the consumer plugs in the 6 pin cable and disregards the published requirements the manufacturer is not at fault.
So what is the answer? You should not use a 6 pin cable in a 8 pin socket UNLESS you know that the card supports a modified power profile based on the sense pins. Having said this, the choice is ultimately up to you and you may encounter no problems whatsoever - regardless of the fact you are exceeding the specifications. Or in a worst case (and admittedly relatively low probability) you could fry your motherboard and/or start a fire.
To put things in perspective, thousands of people exceed the maximum limits for strings of Christmas lights every year. Most people never have an issue. A very small number of people have their homes burn down."
