I don't think using a Slaying ring is worth it at all. This isn't pre-3.2 where it was a big HP loss for a big DPS gain, right now you gain almost nothing. Going by the stat weights on Ariyala (even if they're a bit out of date, they'll be close enough), the Slaying ring is only 8 effective STR/VIT above the i270 tome ring, or 14 above a pentamelded crafter ring. You'll have over 5400 effective STR/VIT at i270. That's a 0.15% increase and I really doubt you'll get more than a few points of DPS from it. An extra 1000 HP isn't going to be a huge help to your healers either, but I'd still rather have that over a DPS gain that's smaller than what I could get with a good run of crits.