Quick question about that chardev; Why Gendarmane's cuirass? Sure it has more str, but the other itemisation leaves much to be desired (I had considered it for my retribution BiS chardev, but it turned out that the tier was better, simply because it provided haste).
My first choice would be gloves and helm. Reason being, the 3% crit damage meta provides a 6% damage boost to crits (it gains the +100% bonus a crit gets), so is likely worth more than the mastery on the 308. Gloves are simple. There are no 300+ ilevel str gloves, and as such the best we can get is the redsteel gauntlets, which only permit an enchant of +44 ap (or 10 haste, lul). This means the 277 tier gloves are much closer, despite the itemisation (hit/crit if I remember, but I could be wrong).
My second choice (if you did not want to drop the mastery from the 308 helm, the trade is quite large for a meta (~100 mastery and quite a lot of str) would be legs/gloves. Reasoning for this is that the non-set legs (whilst fairly awesome), do not provide mastery innately, and as such can be swapped out with (less) of a loss compared to say...Shoulders, which are near-perfect itemisation. The legs are crit/expertise, which again is an issue (damn accuracy stats), but I think you might end up better off for it.
Finally, my third choice would be to use chest/gloves, because you are likely to end up gaining itemisation for this. The tier chest is crit/haste, whilst the Cuirass is crit/hit, and like I mentioned earlier, haste is a decent enough reason for not using the offset 277. Personally, I'd think this option is the better one (certainly better than the 2nd choice I mentioned above), just because of how strong haste is for a level 80 dk.
Overall, I doubt there is any competition for the first piece of tier (gloves), just because we can't really get a decent alternative (Fleshrending gauntlets are good, but I don't think crit/haste itemisation makes up for no set bonus. Shoulders aren't really worth it, because the 308 have very good itemisation, and you are likely make better use of the pieces elsewhere.
Feel free to math in contradiction, but that's just my initial observations.