...in the good old days, a UART (8250) uses a clock of 16-times (?) the baudrate. So ist can sample the signal feg. at the middle of die time.

What is about saying:
1/8 of a bit jitter in 10 bits ? (8N1)
It would be 1,25% error.

I think it belongs to the intelligence (!) of the receiver to take the best samplepoints inside a serial character.

Modern PICs are able to get very close baudrate by using higher frequencies and bigger dividers.

If you are using an PIC-PIC-conversion, you are able to define your own baudrate by using "good" dividers of a standard frequency.