Truth be told, it's not a great mastery. If cunning is supposed to be the assassin/burst champ group it should.... add to that somehow. The extra damage of Thunderlords will ALWAYS be more useful than a MS boost after dropping burst. They should change Stormraiders entirely. Maybe to something like this...
Intimidating Display: Champion kills or assists reduce the armor and MR of enemy champions nearby by 10% for 3 seconds. This ability cannot stack, but multiple kills or assists extend the duration. This ability DOES stack with % armor reduction.
BAM. A tool for champs who specialize in cleaning up fights AND the return of % armor pen instead of % BONUS armor pen. And now it's worth taking in some matchups.