I have been using the UART mode of the SCCs on the '360 for a few weeks now. I am finding that when I get to data rates of > 500 kbps or so, the RISC starts having problems keeping up with this quantity of data on the receive end. (ie. I start getting frame errors or overrun errors).
The performance data in Appendix B of the 360 manual claims the UART mode is good up to 625 kbps.
Have you had any success/failure with higher rates?? Is there any performance difference between different SCCs?