Rep shield and Rep neck would be very good upgrades for you. Not sure If I would have bothered trying to get socket bonus on the legs, I would have just went all stam personally. I do not know what kind of cash flow you are usually sitiing on, but you might look at some crafted epics to give you a boost....helm of the stalwart defender, gyro balanced khorium destroyer, bracers of green fortress. With nethers being much easier to acquire now, a few pieces here and there might go a long way for you.
You can afford to lose some of the defense you have while still maintaining your Defense above 490.
1) Replace your shoulders with the Gladiator's Plate. Very easy to obtain in AV. (These, IMO; should last until the Warbringer Shoulderguards which you won't get until you venture into Gruul's. 2) Do the quest for Shadow Labs and get the Sha'tari Wrought Armguards. Another easy quest. And while you are in there, get the Adamantite Figurine. "Trinket" 3) Drop the gun and get Felsteel Whisper Knives. The HR and the extra AP will be important for agro. But if you wish to stay with a gun; or bow for that matter; try getting a faster weapon. A slow weapon; if not shot at the precise time; can cause a wipe. The Boomstick from Seth comes to mind. 4) Get rid of the Agi gems and the +9 Stamina gems put +12 Stamina in your Felsteels. +4 Dodge and the +3 Parry bonus is not as important as Stamina at this lvl of raids. Just keep in mind you may have to throw in a +8 Defense gem to maintain your 490 rating.
One last bit of advice. I found that rage can be a problem if you don't have your avoidance set right. Too much Dodge at this stage will cause the mobs to miss, which builds zero rage. And unless your HR is high; Parry is better at this stage. I wouldn't worry too much about Expertise. (There really isn't enough gear to give us Expertise until you get into T5 gear. That'll come later.)
Chances are. (a) If you lose agro, you may be partying with someone (DPS) who is over-geared for you. (Don't laugh. I have partied with a T5 geared players when I was just entering Heroics and because of their ability to crit so easily; I couldn't hold agro long enough.) Or (b) you need practice.
Thanks. I wasn't trying to go for > 490 defense, I just kinda ended up there following the pre-kara gear list and hadn't started tweaking the set. Appreciate the feedback on the gear upgrades, I'll give them a hard look.
I definitely hear what you are saying about rage starvation, pretty sure I've experienced it doing dailies... I've finally sort of got a farming set which drops def/armor etc.. so I can get my dailies done in a reasonable amount of time.