Yeah. There are SPI drivers for a similar RAM chip in the code repository that claim to be able to read at a 10Mhz clock and write at 20mhz, and the "FemtoBASIC" package already supports using 23k256 for program memory, so I have reasons to be optimistic. (It looks like an instruction fetch on the z80 takes four processor clocks, which means in theory I only need about a 4Mhz sustained bitrate to run "all-out" at the TRS-80's clock speed. There is of course the overhead of encoding the read-write addresses into serial and sending them, which is why I'm thinking it might be worth trying to do fetches on a page basis and cache.) It just get the feeling sometimes that the "emulator guys" on the Propeller forums are hung up on the "idea" of serial ram being "too slow" and concentrate on throwing hardware at the problem. (Witness monstrosities like the "Triblade Prop" board.)
If I can't make it work then I'll probably just cook up an SD card interface and play with FemtoBASIC for a while.