the corsair psus do have the 8pin mobo power connector. i saw a review of a board think it was a gigabyte one that manage to pull enough power fromthe 4 pin cable that it ended up melting the plastic over it and melted part of the 4 pin power connector socket. 8pin is probably safer to use now since it spreads the load over more cables so less chance of meltage.
the corsair 520hx and 620hx deffo have the 8pin connector, i have both a 520 and 620w and they both have that 8 pin connector, now that i recall i think even my corsair 450w has that 8 pin connector too.