If you're planning on swapping after Unchained, you would probably want to wait until after the swap to use Shadewalker on your DRK. Your WAR and DRK will come out of the swap neck and neck on enmity, which is going to put a cap on your WAR's ability to use BB. In addition, this approach still forces at least 1 PS on DRK following the swap (as a continuation to SpS), even if you sync up your combos and execute it flawlessly.

When you do comparisons like these, you're not really trying to maximise enmity. You're just trying to create a safe enmity gap between yourself and the next highest player. The problem is that this usually isn't a dps. It's your WAR.

I like the Unchained opener, but I'd rather see it used if the WAR is planning to continue tanking to the next natural swap. A5S is a good place for this, for example. Swaps, in practice, are associated with dps losses of their own that need to be offset, so it's probably better for most groups not to swap unless encounter mechanics force you to.