Interestingly I think the whole mastery sub-focus idea holds more water in DS than it did in previous tiers, for a couple reasons:
1) the fairly excessive amounts of raid damage across the board means that you'll get extra rage from THAT and so maxing your rage gen stats (hit/haste) seems a bit less necessary
2) due to the vast quantity of stats you actually have on DS gear, it's probably easier to hit a place where mastery really starts to be valuable anyway
Also, sort of a 3rd semi-point is, mastery is more valuable for arms, so it makes the pain of reforging between the two less.
Still, probably worth testing out and seeing how it goes. It's late enough in the life of the instance that I'm sure there are plenty of farm fights to play around with.
Oh, wrt weapons, the only thing that's better than Gurthalak is a better version of Gurthalak...use both until you get a better version of it. The proc is absurdly good.