Pardon me if this is a stupid question, but I assume that the required efficiency excludes CPU miners, is that correct?
I believe so (effectively), from what I've read so far. But not due to efficiency; instead, due to Stales.
That makes sense, thanks for making it obvious.
Someone should start a CPU-only mining pool. ;)
There is no need for this. CPU miners can work fine with usual pool and get MORE reward than from special CPU-only pool.