As abilities do not have effects when their Pokemon is not in play in a battle, (except Regenerator,) reviving Pokemon can only take away the boost.
The game simply checks how many Pokemon in your party are fainted, adds 10% for each, and applies it to the base power of Kingambit's moves
You can see that this ability acts differently than Last Respects, as Last Respects registers the "boost power" flag 'every' time a Pokemon dies, whereas Supreme Overlord only registers 'each' fainted Pokemon