Theoretical maximum baud rate at 64MHz will be 1,000,000 baud ( 1Mbps ).
Though receiving 1 million bits in a second ( 100,000 bytes per second ) is unlikely to be achievable, with suitable pacing ( gaps / pauses between each byte sent ) it should be possible to use 1Mbps comms. Not that it would really make much sense to do it that way in most cases.
Using the 20X2 background receive buffer it is possible to send at any allowable baud rate and receive back what was sent by linking TX to RX in what's commonly called 'loopback' mode. This allows various baud rates to be tested and you can look at the signals on TX and RX. You can add lengths of cable between TX and RX to see what effect length of cable has and this way determine what issues may occur between two 20X2 over that length of cable.
Internal oscillator drift is an issue with serial comms but in general serial comms is tolerant to around +/-6% combined baud rate error of transmitter and receiver. If the transmitter is 2% fast and receiver 2% slow that should still work ( 4% combined error ). This is regardless of actual baud rate, fast or slow, though it depends on how the receive is implemented; bit-banged reception may require lower error than an on-chip UART baud rate error at higher baud rates. Distortions of the signal caused by cable will also create some error in itself.
Internal oscillator error comes down to manufacturing accuracy, natural drift and change caused by temperature and voltage. If the later two can be controlled there should be very little change in any error other than through natural drift. They often cannot so devices with external crystals or resonators would be recommended for reliable communications. In most cases there usually is no problem, but a comms link from indoors to outside where temperatures may vary widely ( -10C to +40C say ) and power comes from unregulated solar panels or batteries which deplete over time and problems are far more likely to arise. It is however possible to adjust baud rates to compensate if there is no rapid change in baud rate. Easier though to use external crystals and resonators to avoid all that though.