From what I found out on my sumn. 100ss increase means 1 more attack every 2min of nonstop attacks. The effect of spell speed on cast time is 2843-(ss)~round to 3 digits and move the decimal. so my ss right now is 441 so. 2843-441=2402 rounded to 240=2.40cast/recast time. So in the end i would give up ss for more damage stat.