you have up to L45, but it gets better after that.

the shaman also has a cast spd cert (L30 and 60)

that with the illu cert and ice meteor u can easily hit 20%+ cast spd and instacast since the bomb only have 1.5 sec cast time. i personally would put the shaman cspd cert over the stun cert but thats just me.

yah thats just up to lv 45 and cast speed will take a priorety as things move along the stun was mostelly just a filler to get the 4 ranged attack combination +5% attack and magic attack bonus