you're not gaining anything by having more threads than your PC has CPUs

Are you sure about that?

Casual observation seems to indicate that the encoding rate in fames per second on any of the threads stays the same, whether there is one, two, three, or four threads running simultaneously.

So it stands to reason that if I can run four equal-speed threads simultaneously, that should encode things about four times faster than running single threads sequentially.

Or am I missing something obvious here?

tanstaafl.