Why is your expertise so low?
I prefer the massive increase in mainstat over the PvP armorings. After a lot of experimentation I have found that it simply works better for me, regardless of my enemies' expertise ratings. It is personal preference, but I can with absolute certainty that I always do best when it is so "low."
I wouldn't go much lower than 1150 though, for dps assassin anyways.
ALSO I UPDATED MY ARMORY SO CHECK IT OUT
also I posted a thread in 4 subforums yesterday (this one, shadow subforum, PvP subforum, and The Bastion's subforum) about why DPS assassins can be good in rateds. Has some important points that relate to the class as a whole and how you should play it in PvP so I strongly suggest reading it. I will add its 2300-word content to this guide once the actual threads die out.