You should be able to sustain a burst @19200, although your throughput will be lower than 20kbps. However, since this is at the low end of acceptable baud rates to use for bursting, you also need to make sure that your firmware is providing data quickly enough to avoid starving the burst. Some strategies to alleviate the effect of a slow serial port, like making use of the transmit queue, are described in AN04 Burst Transfers.
