bolteon:
Curiously, the 10MHz boards; they only output properly in the serial output when mismatched. Either in the boards.txt at 16MHz and baud at 184320 (115200 * 16 / 10) or the inverse with the boards.txt set to 10MHz and the baud at the standard 115200.
What do you expect to happen when you specify the wrong F_CPU in boards.txt...? F_CPU is used to calculate by how much to divide the system clock by to get the UART baud rate, so specifying the wrong F_CPU will throw off UART baud rates. It will also throw off millis() time keeping. Heck, it could even reduce ADC performance by resulting in the wrong divider for the ADC clock being selected (depending on the two specific speeds in question).