I think that the clock speed has a much bigger impact on power efficiency. High clock speeds are less efficient. So reducing the utilization could be beneficial to power efficiency.

Yes, high clock speeds are less efficient. That's why I don't let my Ryzen 5700G boost beyond 4 GHz manually via BIOS. The MT-tasks do not downclock your CPU, actually it can boost a lot more compared to other projects because it doesn't run into any limits as fast as on other projects. For maximum efficiency you want low clock and you want to use every cycle, not cores doing nothing part of the time at high clocks and voltages.

The kind of efficiency I care most about is power efficiency. If MT wastes power, then that matters to me.

It always does, the synchronisation between the threads is additional work. In addition to that, cores are not loaded 100%, there's lots of waiting, you can see that in task manager.

Have you determined the credit rate with an ST task running on every core vs the max number of MT tasks running concurrently?

Yes, runtimes and credits for 300 tasks of each were recorded and the credit per day calculated based on those tasks.