I'll weigh in here, although the thread was intended for Haswell cores.
I recently tried this with my SB-K, and the limit for either "auto" or fixed "stock" voltage seems to be 4.2Ghz. In the fixed vCORE case, the voltage is set in BIOS such that the actual vCORE shows at the same value given by either the BIOS monitor or software under "auto." But I have done preliminary testing under "auto," and that seems to be the limit.
Since there are two generations difference between the Haswell and SB-K, and since so many people here report 4.2 Ghz, one might ask why. I haven't the slightest idea, though.