What I'm talking about specifically
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Optimal Power is not maximum power. It still uses GPU Boost 3.0 as normal. So its not a fixed clock or voltage in any way and the card will clock down too. Prefer Maximum Performance is closer to maximum power than 'Optimal Power' is. I believe the difference between those is mostly that Prefer Max Perf always tries to go back to maximum voltage whenever there is headroom. If you are thermally constrained, it will do more 'yo-yo'ing' between clocks. If you're not, you can probably extract somewhat higher boost bins.
Still the preferable setting is 'adaptive' because it will use a much wider range of power states depending on the current load. Optimal Power just pushes the highest power state clocks regardless of load and then finds equilibrium and tries to hold it.
The three options are pretty close in practice, especially if you are already thermally limited, but Adaptive is easily the most effective one. Its not a good one for benchmark scores though.