I did some napkin math, and there doesn't seem to be a situation in which the new Consuming Pact makes sense to use.
I performed some tests with my wife's shaman (Level 52)
I based it on a two minute and 40 second period, which is about the time it takes to cast Pact 6x and wait for the debuff to fall off so you can heal.
My math is as follows, sorry for any mistakes:
Consuming Pact costs 162 hp for 182 mana. (At her level)
Consume IV costs 528 hp for 360 mana.
Six casts of Pact is 972 hp for 1092 mana over 160 seconds
Four casts of Consume IV is 2112 hp for 1440 mana over 160 seconds
So Consume IV comes out ahead with enough extra mana to heal and balance out the health cost difference, WITHOUT the massive risk of having to sit at 33% hp for two full minutes.
Old Consume is the clear winner, and this doesn't even take into account how much you lose from the healing debuff reducing other incidental healing sources. (Regeneration, HP Regen items, Paladin group heal, bard heal, etc)
TLDR; Old Consume is better than Consuming Pact in every way