I don't know what could be the "top speed" of the serial link, but I think you can achieve 115000 bauds at least, better than a 56K modem. 
I was thinking on include a plip port for higher speeds, but It would be less compatible, but It's still possible.
The Amiga serial port cannot achieve more than 56kb on 030 or better accelerator. Pure 68000 runs OK at 19200 or a bit over this using software serial replacement like baudbandit or new8n1.
Anyway, this project is very interesting and I really love to see the schematics or the prototype running.
My congratulations to you!
