janost:
What is the actual range of the OSCCAL register?
The datasheet just specifies the range to 7.3 - 8.1MHz and recommend not to go outside that range.
It actually says: "Calibration outside that range is not guaranteed"
janost:
What is the likely resulting clockfrequency from writing 0xFF to the OSCCAL register?
The oscillator might stop oscillating.
janost:
Can I count on somewhere around 16MHz?
No.