User blog comment:Gerudo74/Swain in Season 3?/@comment-24.61.51.11-20121125053516

I am fairly certain that it would stack multiplicatively and not additively, because it makes spells deal 20% more damage, so if you had an ability that only dealt with a bonus percentage of damage(like Poppy's Diplomatic Immunity), it would multiply that percent instead of adding onto it.

I only say fairly because

1. I don't have PBE access so I can't check it and give you an absolute final verdict

2. I REALLY want it to stack additively so I can play me a mean Poppy

Either way, I think they've made DFG incredibly core now(I never bothered with it, but for it's new stats and that new active, I'm all over it), which is definitely what they've set out to do.