I have definately seen people knocked back more than 20 yds by thunderstorm. This would probably be because when I knock them back with the first one, if my second one hits them when they are 9 yds away from me, they will fly back another 20 yds from that point. If they are in the air the will go slightly further. I like to change it up where I either use them all at once for massive dmg, rotate over a period of time to keep knocking them away and chaining them for massive damage and potential extreme knockback :P And of course, over cliffs is my favourite situation lol.

PS: I'm surprised keyclone and Fur haven't jumped on you already for creating a new thread on this topic when a few have already been made from when the spell was first introduced, hurr hurr hurr.