An update... Ran the previous tank nearly dry (!) and refilled with 91 or 93 stuff (as measured in U. S.; the manual says 89 AKI or 95 RON), and have run the car ~275 miles since then. Haven't noticed any pinging in situations where I did before, and subjectively the car seems to have more power. It could be one of those quirks where at least over the summer, and perhaps due to carbon buildup, the higher octane is what I need, although I'll do a bit more experimentation.