The 40% yield refers to number of good chips per wafer - not cores per chip. As to what is a good chip, it one that meets spec. Preferably with 100% cores active @ stated speed. From there they will start stepping down the grade hopefully mainly by speed capability instead of by dead cores.
You start sounding weird... Not enough caffeine this morning? Or too much?
The whole point of mining ASIC is that there's no "spec". It is 100% self-love. All they have to do is twiddle their own little thumbs really fast and gaze at it's own navel. The theoretically most demanding application for them would be if they are daisy chained and have to talk to their brothers from the same wafer or same batch that is sitting few centimeters away on the same board. No need to interface e.g. a DRAM chip or obey some IEEE standard.
The only 3 things that miners care are:
1) Ghash per sec/W
2) Ghash per sec/$
3) delivery time
There's no "stated speed" or "100% cores". Everything else can be and will be worked around at the board layout and software driver level.