Healing threat is insignificant and shouldn't be relied on, as it only does 1/2 the threat of the EFFECTIVE heal...meaning even if your DS would have healed for 6k (which is only 3k threat anyway), if you only got 500 health out of it then you're only getting 250 threat value, overheal isn't counted. Furthermore, unless you're using a blood spec, Death Strike will be a waste of the FU runes, since you won't be getting conversions to death runes for additional buffed strikes. If you want to use death strike routinely, switch to a blood spec (which I do see you say you did, so that works out).