Your dps should be a few 1000 higher when u get 4set bonus, and more haste on gear, so u could gem intellect.
Also don't think of mastery as a very bad stat, it is only very slightly behind crit.
And.. 100k dps mage with 502 ilvl gear is far from good. Their single target dps is insane right now, but u should outperform them at fights with additional targets.
For exmaple my 500 ilvl priest is doing 90k dps on Bladelord, but on Will of emperor he s capable of doing 130k dps.