Mike,
This is going to be really exciting. More performance = more possibilities, just look at PreenFM project based also on STM32 chip able to do some 4+ operators FM.
I have been working with Atmel and ST microcontrollers for a while, so would be happy to be of any help. (And a bit of FPGA, too, if you decide to replace oscillator chips with anything more up to date :))
BTW there could be at least one concern, with new CPU replacement being too fast to drive old peripherals (I doubt the rest of Poly 800 could be clocked higher than 2MHz; the caveat I have discovered once trying to mate 72MHz STM32 micro and some retro IC that would not run faster than 4MHz)
Regards
Tim