cmaglie:
The Serial class uses the following formula to obtain the clock divider:_pUsart->US_BRGR = (SystemCoreClock / dwBaudRate) / 16 ;in your case the perfect divisor would be 84000000 / 10400 / 16 = 504.8076
but the register will truncate to the integer 504.
The error is under <0.2%IMHO you should give a try with Serial.
I agree with this assessment. Failing that, perhaps the Multi Serial shield would work...