If anyone still on the fence and wondering what his masteries are:
Power Set 1:
Increases the rate at which indoctrinated laborers and troopers increase imperial support
Increases the rate royal guards increase imperial support
Power Set 2:
Increases damage of contaminated strike, nuclear annihilation, and zerg deployed by dogs of war
Reduces cost of royal guards
Power Set 3:
Increases initial imperial mandate
Increases the rate at which royal guards gain experience