Here's how I think of it:
Vidle = VID(1.6 GHz) + Voffset
Vload = VID(4._ GHz) + Voffset - Vdroop(LLC)
- VID(x) is a look-up table (or curve) unique to each chip, found through experiments.
- Vdroop is varied via LLC, but with limited control (only 5 settings, which gives limited precision and maybe not enough range).
But if using Offset mode, be prepared for more work, confusion, and a good chance of no overall improvement vs Manual mode. With Offset, you have to get it stable at both idle and load, you don't know the VID curve upfront, you'll need to play with LLC and Voffset simultaneously to get Vidle and Vload to both be appropriate, and you'll have limited control on optimizing Vload.
With some chips it may indeed offer a good reduction of Vidle with an optimum Vload. With other chips (I think most), Manual mode will give the lowest stable Vload, and idle is guaranteed to be stable.
If a stable OC is achieved using Offset, you might try switching to Manual to see how low a Vcpu you can run the high speed stable. That Vcpu is likely to be lower than the Vload using Offset, since you can optimize it independent of idle issues. For chip life, I'd would think that a relatively small drop on the high end is better than a big drop on the low end.
For power efficiency, my kill-a-watt meter showed only a 2 watt difference for wall power between a 0.9 Vidle and a 1.25 Vidle. (At idle, there is so little current flowing that it doesn't much matter what the V is.) There may indeed be a larger difference at a mild load, but I'm not sure how to rig a constant mild load that doesn't raise the Ratio--any suggestions?
If Manual mode tuning resulted in a Vcpu that was so high it may shorten longevity (1.35? 1.4?), then I'd look pretty closely at Offset mode. But I'd keep it only if the resulting Vload was the same or just a little higher.
I really wish the BIOS had a more intelligent method for controlling Vidle and Vload, like setting them independently and interpolating, or having 2 offsets: 1 for Vidle, 1 for Vload, and interpolating on top of the VID curve. That would be sweet! I wonder why they haven't done it.