2x 6-pin PCIe cables are used to power my Bitfury M board.  A few people have had their's go up in flames when using between 500-600w.  The three 12V molex contacts on the 6-pin connector themselves are good for 300W or so all combined, so long as your PSU manufacturer doesn't cheap out on them and use a smaller gauge than spec (which seems to happen on occasion).

It shouldn't be a problem if we're all getting seasonic psus.