A new version of the A-Tuning utility was added to the download page of the Z87 Extreme 6 on 7/12/13, and also for many of the other ASRock Z87 boards. It can be found in the "Beta Zone" link on each board's download page, most boards have their own version.

I have not used it much at this time, but it has some obvious improvements. The System Info screen now has both CPU Input Voltage and Vcore Voltage displays that are (seem) correct. The CPU core voltage settings display is improved.

The only thing I can find that is unique to the OC Tweaker feature in A-Tuning that is not available in the UEFI, is the CPU Input Voltage Offset can be a negative value. You can reduce the CPU Input Voltage up to -0.600V. The UEFI only allows pre-programmed positive values in the CPU Input Voltage Offset field.