It looks like the M1 does run all out all the time (maybe because it never turbos that much to begin with).

When plugged in, perhaps, but when on battery those slower cores are going to get used more.

No, if you run a heavy computational load on a battery powered Macbook Pro it runs all out on all cores. Lots of reviewers looking at this. Battery life is worse of course under heavy load but still incredible relative to x86 for identical tasks. Faster and less battery usage.