Again, second sourcing is itself not the problem. Second sourcing with a part that is 20% slower than the alternative is the problem. Show me two sticks of ram with the same specs that have 20% different performance. You can't, because there is no such performance difference so second sourcing is irrelevant.
Whenever there is a 20% performance difference I guarantee you that some subset of enthusiasts is going to care and will return return return until they get the faster one. And I don't understand why anybody is outraged by that.
At the very least, Apple should have tested the products from their suppliers before approving it for usage. For instance, Apple should have benchmarked the SD0256F and rejected it for use in the retina MBPs because performance isn't consistent with other parts.
On the other hand, the SM0256F performs very closely to the SM0512F, so generally it can be said that Samsung parts are pretty consistent.