In case anyone is wondering, I believe I've found the answer to my question.
The -40 and -41 version US Sunny Boys are compatible with CA Rule 21 and UL1741-SA.
Up to now I hadn't paid attention to the CA rule because, well, I'm not in California!
It turns out these "extensions" to UL1741 (anti-islanding) add requirements for frequency shift ride-through, among other things.
And they include the optional "frequency-watt" control, AKA freq-watt or what SMA terms FSPC.
Phase 1 of Rule 21 was enacted in CA in late 2017, with Phase 2 (monitoring of your inverter by the grid operator) and Phase 3 (control of your inverter by the grid operator) coming later (2018-2019 I think).
I'm not sure which, if any, of the other 49 states require this stuff (Hawaii has its own very stringent requirements).
So as I now understand it, the newer versions of Sunny Boys will happily implement FSPC, if they are set for Rule 21 operating mode, regardless of whether they are connected to the Sunny Island AC1 terminals or directly to the grid. This means no parameter changes are required when switching back and forth. That's as far as I understand, but I still haven't heard anything authoritative from SMA.
I'm still not sure whether this might have been an issue, requiring parameter changes, for pre-40 Sunny Boys used with Sunny Islands and the E-Panel bypass switching. Because in previous models of Sunny Boys, absent the RS485 comms from the SI, a slight increase in line frequency (say 1 Hz or less) would shut down the SB output entirely. Perhaps tripping the bypass switch would have stopped the SI from telling the SB to go into full island mode?
I suspect this stuff is all "old hat" to quite a few active solar installers; but for me, I'm still learning about it.