Someone did some fairly comprehensive testing for Skillspeed a while back and learned that 72.5 Skillspeed is 1% more damage from DoTs. There's little reason to assume Spellspeed works much differently. SS is still garbage, and that's not even counting the messy rotation, clipping of DoTs for classes, or added TP consumption -- it's just notably smaller boost than det, crit, and certainly accuracy to cap are.