Seriously the difference between LFR/Normal/Heroic gear in the same tier is MASSIVE. Let alone the difference between a previous tier or two in gear...
In TBC on my mage I i don't remember 100% exactly how the gear/dps difference was....but ill try using my foggy memory...
kara - <1000
gruuls/mag - 1000-1200
ssc/tk = 1200-1500
bt/hyjal = 1500-2000 (i actually have a fraps of our first teron gorefiend kill which I did 1800 dps on, so i remember this 100%)
sunwell = 2000-2500 (best I did pre3.0 was prob 2900 on brutallas with retarded high crit ignite dmg with drums)
I also remember the shadow priests in the guild always complaining about how their dps barely went up from tier 4 to tier 6.....because of the way their dps worked. They went from like 800-1000 dps in kara to doing like 1300-1800 in sunwell, which was way behind the other dps classes.
Man I am disappointed in the squish...I was it hit even hard...like back down to Vanilla/TBC levels.