I tried using
https://wiki.project1999.com/Torrent_of_Poison for a while on my Shaman thinking the same thing (good mana to damage ratio). Not sure how consistent this is across rain spells and mobs, but I found that individual waves of the rain have a pretty good chance of being resisted. Enough so to cause concerns about drastic reductions in the damage per mana ratio. If you have one wave of the rain resisted on average, that is a big drop in damage per mana. This is just my own anecdotal experience, I did not do any parses. I did try the spell on quite a few mobs, including greens when I was farming high quality tundra kodiak pelts for the Thurg Ring Quests.
But I would try doing some tests on your end. Just make sure the mob doesn't run up to you, or accidentally target yourself. I have killed myself once or twice with Torrent of Poison lol.