Definitely not necessary at all. I go for 30% spd for 0.8 second DoTs. But as you can see even with 30% speed I am able to spam all my skills (especially soul draw) quiet frequently. If you so happen to have gun or bow buff in your party, you'll cap or near cap your speed anyways. It's much better to invest those extra stat points into other stats. 20% more speed really doesn't make much of a difference for sorcerer, most of their skills have low cool downs anyways.