It's much more simple than that. AMD stated that 1.55V was the maximum that their 45nm SOI process should be subjected to. It hardly matters which core you've got. 1.55V should be safe provided the silicon is kept in check thermally (at or below 62C) and 1.55V is what your board is actually giving it. Load line calibration can cause major spikes, so it's worth being mindful of that. Numbers above that are just speculation. Numbers below that are just preference.