> Using an expensive GPU block (with most of its cores idling) to encode slightly faster than a cheaper CPU core will do?

Slightly faster? Sir, I do think you are mistaken.

There are other variables go that into it such as choosing between constant or variable bitrates & variable or constant frame rate, and profiles obviously. There's a crapload of other directives that you can tweak as well.

I see what you mean though, you're right if you have CPU with lot of cores/threads/horsepower then you should use that instead of hardware-accel transcoding. I don't.