Are you sure about that? My background is limited but my understanding is that chip makers will set their spec limits far higher than need be. The chips then get tested, and those that struggle at the higher frequencies will be designated as the lower end processor. Or those which have a single low speed core will be designated as "tri-core" processors.

I know these practices wouldn't apply too well when you're making a PS4 with a single performance level, but that 60% yield still sounds horrible from a process design perspective.