Its no bug, the OS will always load the OS MS mcode file on top of the hw mcode, so it must be disable.
These mcode are loaded in the file mcupdate_genuineintel.dll, rename it or delete it.
A thread on old mcodes: [OFFER] Intel CPU Microcode Archives - Special Topics / CPU Microcodes - Win-Raid Forum (level1techs.com)
The Version 19 Date 13-06-2013 is considered to be the best for 306A9 (IB) regarding OC values.
The Version 28 Date 24-04-2012 is considered to be the best for 206A7 (SB)
EDIT: Whats the difficulty here…If the HW (mcode bios motherboard) is superseed by the OS MS file mcode when its loaded, so disabling the intenal OS mcode file, then remains again the HW mcode… easy enough now?
Related to performance… i only shared with you, whats is the best shared opinion around the web from IvyBridge OC mcodes, your decision on what mcode to use or the most suitable for your needs, every user should do his own tests.