Burst damage cap increases stack additively
They cap at +100% (thus raising the burst soft damage cap from 1 million to 2 million)
Burst streak bonuses are more interesting at high enough power levels where the 2 million cap for burst damage is your current obstacle. That's the point where you're busy looking for ways that get around it like burst streak (which is in itself uncapped as far as I'm aware) and/or echo bursts (extra independent hits that exist outside of the burst streak technically). But until then, you're more concerned with other stuff.