Teleport Mastery is

This reason alone is why I can't see it being useful for increasing bossing DPS. Chances are it would screw with spamming main single target skills too much that it won't be used.

*Imagines 6 Magicians teleporting around the bottom of HT's cave to kill Dark Wyverns*

^ Useful....hmm.....

Well the delay only occurs if you actually hit something, otherwise it's just a normal teleport. The lag i experience could easily be because of the cross-continental connection too though, so who knows. :/