I've read through the forums & guides here with regards to baud rates and processors, and the fact that it isn't just the baud rate that should be taken into account. But, I can't find (or work out) exact figures here.

I know that to get a required baud rate of 115,000 on an Atmel AT89S8253 I'm going to need a 22.118 crystal, X2, and TH1 of 0xFF

However, although that makes 115k baud possible, how viable is it to actually do any other processing at the same time?

I need to sit on a very busy 115k multi-drop 485 line, and be able to receive variable length packets, and respond accordingly, whilst also monitoring various other inputs.

From my figures (which may be flawed), there doesn't seem to be much time in between the serial interrupts to actually do anything else apart from process the incoming bytes! Especially when I plan on doing this in C so cannot optimize the assembly code apart from Keil's creation.

In which case, what kind of speeds would be more appropriate for this kind of baud rate & operation? Or would a different chip be a better choice?