The max baud rate will depend on the capacitive loading on the voltage divider tap. If, for example, you were to put the divider at the TX, and the RX were to be hundred feet away on a twisted pair that has a capacitance of 50pF per ft, then you would have to consider the low-pass filtering effect of shunting the voltage divider tap with 5000pF. A sim of this case at 115KBaud shows the voltage at the RX as a function of stepping the resistance values in the voltage divider as R = 500, 1K, 2K, 4K and 8K. In the example, I would say R=500 and 1K would work, but R>=2K is getting marginal. You will have to recalculate based on a realistic estimate of your line capacitance.
ps, it occurs to me that the RX has a line terminating resistance which appears in parallel with R2 in my schematic, so you will have to consider that as well. Also, if you put the divider near the RX, then the line termination could be combined with the divider.