Little discussion as been had about this, but it seems that the Cunning gain from the 78 armorings now closely balance to the PvP 2pc. The PvP 2pc still comes out ahead, but only if you're able to get in all four ticks. Once I've gotten to run DF and DP HM with Goopy I'll be able to comment directly on which I feel is best, PvP 2pc or PvE 4pc in 78s. My decision will come down to the overall usefulness of XS Flyby against a boss or adds.
With my current knowledge of DF, I'd say keeping the PvP 2pc would be wise as the second and fourth encounter will greatly benefit from additional AOE damage, and the first encounter is static enough to get in all 4 ticks easily. However, dropping the PvP 2pc for the third and fifth encounter may be an improvement unless you're specifically assigned to add duty.