Desperate Prayer where art thou?

[Lissa]

Well, not really, as I said above.  If the mob lives less than 18 seconds, then you had no increased mana efficiency at all.  Similarly, if the mob lives, say 30 seconds, you'll still have to cast SW:Pain on it twice whether you have Improved Pain or not, so again there's no improved efficiency.  There's some increased mana efficiency here, but in real life it's much less than 25%.  My two favorite uses for Improved SW:Pain is on mobs that are running away and on PvP situations where you want to damage someone running away or want to damage someone about to kill you as a last desperate act.
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Actually, if a mob looks that close to death after the first SW:P wears off (where only 2 ticks of SW:P would kill it), I usually do not bother with a second. I would rather take a couple more seconds and just finish it off with Melee in that instance than cast another spell to complete the kill faster. Besides, if I skip casting a follow up spell to kill it, that means I'm regenning mana already before the mob dies and if I have Spirit Tap, as soon as it dies my mana regen rate will double.

In truth though, most of the time I see mobs last about 20 to 25 seconds or last 40 to 50 seconds. In those cases, Improved SW:P is very nice over the normal SW:P.