I asked him if he could replace it with a Samsung (or anything other than the Toshiba with a known problem), but he said Apple will only allow him to order the exact same part as replacement under the warranty programme, which I suppose is fair enough except that it's silly to replace it with a drive known to fail prematurely...?
For what it's worth, Apple will frequently have multiple revisions of a component without changing the part number. For example, component 123-A installed in September of 2013 could be the original version, however component 123-A installed in March of 2014 could be revision 2 or even 3. Very rarely will they inform technicians of any changes aside from "part 123-A revision 2 is available".
If they are seeing large scale failures then they will likely work with Toshiba (or whoever) to iron out any issues either through hardware or firmware engineering and then start shipping revised units for repairs as necessary. If the failures are widespread enough (i.e. more than "half" of shipped Macs are affected) then they may even rollout a repair extension program (REP) to address out of warranty failures, similar to the NVIDIA GPU program with the older MacBook Pros and the top case replacement program for the polycarbonite MacBooks.