What is BiS?

I have a Sentinel with 106% Accuracy, which I think is unnecessary. I would like to exchange some enhancements to replace that Accuracy with Strength or Power. I have Battlemaster gear and I would lose Expertise if I did this, about 200 points. However, I would put at least level 61 mods in there. Is this a good idea?

So I see 350K damage dealt in PvP for other Sentinels, even as high as 500K once. DPS as high as 500. And then there is me...100K if I'm lucky, about 125 DPS. 3 War Hero pieces and the rest Battlemaster, all augmented. What am I doing wrong?