Well, with mage int buff and 10% more mana, you can squeeze in around 7 k mana. That's roughly 10 casts. More then enough to nuke anything. Plus, you got spirit regen so some of your mana will get back to you with all that spirit/mp5.

Also, 400-500 is for healing only, not for damage. Still, that is insane for aoe dmg and healing. Most people will do 400,500 dmg in 2 gcd. So with shield and that damage they will die in 2 gcd.

Also, I did math and HWL gear on locks and mages is almost no difference (tiny) so you're looking at same health pool (no 15% talent and imp) but still.
You can notice that why shamans were that good in TBC? I'ts all about burst damage in this game. As soon as enemy recovers>FUBAR.