Well at my current manual setting of 19x (3.8GHz), each core pulls 1.288V under load according to HWInfo, which corresponds exactly with P-state P2. P1 and P0 are the Turbo settings, apparently, and they shoot up to 1.3875 and 1.425.
Worked a treat! Processor is steady at 3.8, and 3dMark Firestrike score is a good 2.5% better overall, and over 6% up on the CPU score. This will be a very good starting point for further tweaking.
Thank you so much!