Oh dear my brain is starting to hurt. I have upgraded to High Sierra and everything is fine with no APFS related issues to worry about right now.
Could it be that full implementation of APFS might blow away many of the problems which DiskWarrior and other similar utilities were originally designed to fix? In view of the above view that optimising SSDs (and my next iMac will no doubt have one) might actually be counterproductive, I may abandon Diskwarrior altogether unless it comes up with some new and indispensible repair functionality. I'll stick with Carbon Copy Cloner though.
