@Logan: 6% Arp bonus from T9 2pc only comes from battle stance and hence is an Arms bonus.
@OP: I wud personally stick to 2pc t9 till you get 4 pc t10.
IMO this is the helm you should be using (assuming its the easiest to get) Faceplate of Thunderous Rampage. This should keep your hit around 200 and give you 6% more ArP.
The increase in DPS is most likely from you off hand being expertise capped plus more str. I personally would stick to gormok band heroic as ArP should have more SEP at this point for you. I would say experiment with both rings and see if off hand being at lower expertise is hurting you a lot or not.
You should also get berserking on offhand as well.