I'm not saying 5k lpfs is insignificant. LoH, in this build, returns more. If you assume your dust devils hit at least one target, 20k of LoH(bare minimum for an ancient 1H weapon) returns approximately 20k * 3 * APS per second. At 2.5 APS, that's around 150k lps. 5k lpfs, assuming 20 rage per second(which is generous over estimate), returns 100k lps. I think getting both is the best solution since they are different types of healing. But for pure throughput, LoH is better. If I could ever get ideal BK swords(lol, fuck you default socket affix), it would probably be IAS and LoH.
The 3 comes from 1 for channeling WW and 2 for spawning dust devils at twice your APS, each one proccing LoH. I can see how IIB would feel better than LoH since 2/3 of the LoH comes from dust devils. Since you only get the LoH on the first hit, you can spin into a fresh group with the already generated dust devils having hit something, so when they contact the new bunch, they won't proc again.
I don't get your comment on single target. LPFS and LoH are basically both single target, neither scale with targets barring indirect scaling where you might get more resources with more targets and can spend it faster. The exceptions, of course, are proccing LoH with Slaughter or Blood Funnel. Very few heals exist that scale with targets, it's a bad oversight in the game currently. On large enough groups, I've spun through multiple arcane beams and only one non-ancient affix and Slaughter. I haven't been able to duplicate that with IIS. It's a shame Slaughter's single target damage sucks so much.