AFAIK, the extra 2 pins will supply another 75W (max) to the card, a single 6pin can only supply 75W, but a 8 pin can supply 150W.
So, your card has dual 8 pin means it can technically draw up to 375W (150x2 + 75 via PCIe slot). Of course, the card won't draw that much. As you said, it draw just more than 200W max.
So, now, the problem is how the loading distribute. Since it has dual 8pin, if assuming the loading is equally distributed between them, then the max draw will be about 100W on each 8pin.
The cMP's 6pin only rated up to 75W, so, I won't recommend anyone do anything over that limit. However, my real world experience (dual 7950) told me that each 6pin can supply 100W without any trouble. (N.B. The self shutdown protect occur at about 120W).
By considering there will be some power draw via the PCIe slot, the actual power draw via the 6pin will be less than 100W (average).
Therefore, practically, you can just use dual 6pin->8pin adapter to power your card. However, I have to remind you that it's technically unsafe to do so.
Also, if the power loading is very unevenly distributed (e.g. 30W from PCIe slot + 50W from 1st 8pin + 130W from 2nd 8pin), your machine may still self shutdown and may suffer from permanent damage.
A better way to do it may be connect a dual 6pin->8pin adaptor to the mobo, and then 8pin-> dual 6+2pin to the card, which means the 8pin actually serve as a bridge to share the loading between the 6pins regardless how the card draw the power.
