The baud rate is set as shown on pages 20-21 in the datasheet. The baud rate for each UART can be individually programmed, but the process is the same for each. The maximum baud rate for each UART depends on the clock source used, the PLL and the divider.
In order to investigate if the micro can fill and read the TX and RX FIFOs for all of the UARTs as quickly as we needed, this would depend on how you configure the software and how much data each UART needs to handle from downstream circuitry.