I can't confirm anything, but my Cluster bombs skill was doing 1 200 000 - 1 600 000 crits, after I got my bracers with 15% Fire skills damage increase I get 1 500 000 - 1 850 000 crits, so it seems to multiply your total damage rather than just add 10% to your skill. But, again, I can't confirm this.
Use a skill that deals 200% damage as cold. Acquire item that says 'Cold skills deal 10% more damage' and equip it. Am I now dealing 210% or 220%?
Thanks much in advance!