Well im wondering too, some guides say go for haste, but i feel that mastery for me is more comfortable, since haste only gives faster cobra shots and pet faster hits, but till you dont have 2 pieces of t15 it's not worth to go full haste
I have 530 pve gear and don't even have 4set piece, have 3 parts wasn't lucky with chest, but anyways those VP items usually don't have the correct stats so you suffer from those a lot, I'm doing 220k to 250k but it may vary depending on the fight, sometimes its lower depending on a fight.
Mageara is a fight when you switch targets a lot, so sometimes you have to wait for that other head to pop out, basically you lose 3 or 4 sec on every head either you need to run out with cinder or wait for head to pop out, then you have to reapply serpent sting which makes you lose dps.
Your rotation is good, stat priority also, I'm using Crit>Mastery>Haste build but i think either of 2 builds are ok,
what is your latency? that can be a factor in dps too
On Mageara yr pet wont attack (their attacks). If u pay attention yr pet wont use any of his focus or doing white dmg at all. Thats the problem for Bm hunter since our real dmg its from our pet.
To "try" to fix it u have to call yr pet back and attack in the middle of the fight sometimes and suddenly yr pet start to do white dmg and his abilities.
Take care.
ps: i feel more comfortable using crit->haste->mastery prio.