There was shaman called Glyptic on my old realm (Khadgar) and he was a fucking pro with bad gear, so your argument about classes (shaman in your case) unable to get rating is kinda.. meh :/
Btw I also got 1800 in deadly/hateful/pve mix and ilevel 200 daggers in s6, and climbed around 600 rating more later in to the season Playing with equally geared priest.
Glyptic, I'm pretty sure, is like 2600+ now. In any case, my argument is not quite as you make it. Basically I'm saying that bad gear holds back mediocre players (which most people are). Great players will obviously do well no matter what...but those people are the extreme minority.
I'm not claiming to be the best shaman in the world.