@ AndyP

This is quite interesting.

My experience is with a "well calibrated" INTRC-OSC at one end and a Crystal at the other end (PIC-2-PIC communication)
you can get more or less reliable communication even @ 9600.

With INTRC-OSC at both ends 4800 was the maximum I could achieve (reliable).

If either one of the two PICs has a tuned Crystal the Dirift of the RC-PIC doesn't matter that much,
but if they are both RC "clocked" and they drift different ways it does matter.

Please keep us posted.

rgds.