Remember that the fundamental speed limit is the rom access cycle time, which is a minimum of 8 cycles on the 32X, assuming no collisions with the 68000 or the other SH2. Given a 23 MHz clock rate, this means you get a maximum raw access speed of 23 / 8 / 2 ~ 1.4 MBytes/sec. So at least theoretically, we may be able to get some decent speed from a 32X version of the menu. I don't know if the MD Myth cart itself imposes some kind of speed limit on that, but given it works with 32X games without slow downs, I would guess not.
I guess I should probably do some tests to see how it actually works... maybe make a quick test version from the existing menu. Given the extra memory in the 32X compared to the MD, it should also have fewer limits as I don't have to try to squeeze things into 64KB.