Once you manually set vcore, neither C1E nor EIST will affect the core voltage anymore. Currently on bios F6a on extreme, EIST has no affect on the multi (using regular desktop power scheme) but enabling C1E you will see the multi/mhz drop. Also, by enabling C1E the idle amps and TDP/watts drops from 33W/23A to 26W/19A or roughly 20%, whereas EIST enabled there is no drop in watts or amps. However, if I remember correctly on earlier bioses EIST would affect multi to some degree as well, or at least with earlier programs.