I'm quite prepared to believe that better use of memory is possible - the runtime footprint of PHP code seems to be around 8 times the footprint of the script on disk - so plenty of scope for improvement. I was running my tests with 100 concurrent connections - not nearly saturating my machine. I expect that running with a much higher load / less memory would translate into the the performance improvements reported elsewhere - more testing required?
Meanwhile I had another look at the optimizer. I repeated the setup from last time, running with PHP 5.5.1, ZOP+ with filemtime checking off, fetching a single Dokuwiki page. The control test with full optimization - as per previous run - is giving slightly different results than last time. Since I'm running this on my home machine and it's also running lots of other things like X, KDE, my browser, mail client... it's possible that the system isn't in exactly the state it was in when I ran the previous tests.
Optimization disabled :opcache.optimization_level=0
Repeating the test several times gave consistent results; about 2% improvement in speed.
Not revolutionary, but every little helps - and in fairness Dokuwiki already has good performance optimization in the PHP code.
When time allows I'll go back and look at memory / CPU usae while running ZOP+ vs APC