tito-t:
I have no idea how to test reliably, and I am not asking about a test by myself,
Try different baud rates until you get to speeds that don't work.
I am simply asking for the ultimate answer to my question:
The maximum board-specific Arduino UART clock has to be quite close to one of the following (it's limited to this list because the corresponding Raspi supports just this subset > 115200):
I have said before baud rate limits for the Arduino microchips are clearly set out in the datasheets for the chips. For example Table 19-12 shows that a 16MHz Atmega 328 has zero baud rate timing error at 1,000,000 baud
...R