I asked myself the same question and poking around the datasheets I believe that 15V for the main board should be the absolute maximum, with some precautions.
The reason being is that the audio amplifier absolute voltage limit is 15V. I think all the other components can handle the 15V voltage.
This assumes of course that the 5V regulator on the raduino has a heatsink and better also a series resistor to limit it power dissipation.
Before I changed the finals to RD16HHF1s, I added a 12V LM7812 regulator for the main board supply, and fed the IRF510 finals directly with the 16.8V from the 4S lithium battery.
That way you have a maximum voltage for your finals and a safe one for your main board.
With the 12V regulator you only need a heatsink on the raduino 5V regulator (no series resistor) as the heat dissipation is more reasonable.
73, John (VK2ETA)