It sounds to me like this is a problem with:
A) The amount mastery increases item power by (It is significant and punishes you in the short-term for committing to something that gets nerfed)
Go and compare the damage difference of someone holding a weapon in the black zone with 0 mastery vs. 50 and you will see the difference is minimal. You too will find it's rougnly 5%. You will realize just how crap the mastery's are then. If anything they need to increase the gap.