Further reading in the Mac Family Hardware indicates the SE used the C16M clock signal, whereas the Plus was half that speed. This may be the reason people just didn't buy the SE FDHD kit and upgrade their Pluses. Still I wonder if the Superdrive would work at all with a SWIM chip running at half the clock rate on a Plus, or just slowly?

Tom, thanks for the clarification. I thought it was something like that ... so the fabled 256K Mac would have required special decoding to work with the 64K ROMs had Apple decided to go that route with the original Macintosh?

At least I know I can start playing around freely with the ROMs!