What does it matter if our mastery is passive or not? The fact is that the old and new mastery will give us exactly the same amount of interactive play during combat, which is nothing by the way.
And why does it have to hit harder? If it's one of our signature spells, then it should be resposible for a significant portion of our total damage, which it does.
You have two options, less damage but more frequent use, or more damage and less frequent use. The latter is called Chaos Bolt, the former is and always has been Lava Burst. Also, this is from a heavy movement boss with as much mastery as I could get. Lava Burst would have done more damage if I didn't run around so much.