Hi there, first post here, so please be gentle
According to some comments on thottbot, Icy Chill does not stack with Thunderclap.
Assuming the "reduced effect against level 60+" (more on that below) makes it less than 20%, and you have a warrior with Improved Thunderclap in your raid, this enchant definitely isn't worth it.
Edit:
I just went ahead, enchanted a weapon and did some testing against a level 70 player and came up with the following results:
The tooltip says "25% weapon speed", but this is not true, similar to Crusader.
Normal attackspeed of the victim was 1.80, debuffed 1.56.
As far as I know weaponspeed is calculated with
1.80 / 1+x = 1.56
1.80 = 1.56 * 1+x
-> Icy Chill modifies attackspeed by ~15.4%.
But you might have noticed, the attackspeed actually went *down*. It should have gone *up*, right?
I tried to confirm the faster attackspeed with Quartz' swingtimer, and the mage really hit me faster than before.
I also have some screenshots of the character screen and a very clean part of the combatlog (no parries, just hit, debuff, hit, debuff fades) should anyone be interested.
Next I tested (3/3 Improved) Thunderclap, and attackspeed went up to 1.88, so it does stack - in PvP at least.