Hear me out, Strength of the Ages should not have a cap, here's why:

Strength of the Ages implies It gets stronger as the game goes on, and it does exactly that- but there's a cap. If I pick the mastery it's simply because of the 300 health, I don't care about the 6% hp back on every siege minion kill, sure it looks good on paper. But considering siege minions don't come every wave, it's not useful for split pushing. By late game when you reach the cap, most tanks have enough hp regen anyways. Grasp of the Undying blows it out of the water when it comes to sustain dueling.

So remove the cap, but when 300 stacks is achieved cut the efficiency of the stacking by X amount, let's say cut it in half. Let the mastery stack like it's suppose to.