From what i have read over on the house of help site, there where a fair few people who where less than happy with their 6400.
There is a shortage problem with the micron D9's as we know and this lead to the fitting of PROMOS ic's which to be fair are not perfoming anywhere near how well the microns are.
Now i know that if they run at rated then thats all they have to do, but when this becomes an issue is when reviews are done on 6400 micron, then there is an IC change - people see these reviews and buy on the premise that they will be getting a similar perfomance.
This is in no way corsairs fault, micron have the issue with supply / demand.
So you can bet your bottom dollar that the top crucial stuff is running on some sweet micron still.
If you are wanting some overclocking loving from corsair you will have to go over to the 8500 or the rather pricey 6400 C3.
If you are interested heres a link from yellow beard on the corsair help.
http://www.houseofhelp.com/v2/showthread.php?t=44365
This will give you an idea as what is under your heatspreaders.