Note: When I talk about the number of targets, 3 targets might mean 2 people on full and 3 who will get the full heal or 1 person will get the full heal and 4 people will get on average half. I do not distinguish. This is why fractions of targets make sense.Let's assume 1000 +healing (after gear, spirit, buffs everything)
PoH Rank 1 then heals each target for about
321*1.1 + 1000*3/3.5/3 = 639
Mana cost is 328
Cast time is 3s
HPS per target is about 213
HPM per target is about 1.95
If you hit ~4 targets, a fairly common situation, you outpace FH6 in HPS and have the HPM of LH3/H1/H2. At ~3 targets you are comparable to H4 in both HPM and HPS. Above 4 targets the only spell that can really compete are higher ranks of PoH.
PoH Rank 4 is another beast entirely:
940*1,1 + 1000*3/3,5/3 = 1320
824 mana
3s
HPS per target is about 440
HPM per target is about 1.6
If you hit ~4 targets your HPS is about double that of FH7 (max rank) and has the HPM of GH1 (without 8/8). At ~3 targets you are still at 150% HPS of FH7 and have significantly better HPM (4.8 vs 3.75). You shouldn't use this on ~3 targets in situations that are fairly safe unless you think there is a very high likelihood that more damage will hit your group during the cast. Over 4 targets its just more of the good stuff, at the magic full 5 you have 250% of FH7's HPS at LH3/H1/H2's HPM.
Note that all of these comparisons are against the other heals when they do absolutely no overhealing. If you factor in that you might overheal say 20% of the alternative on average, then PoH only has to hit ~3.2 targets to get the 4 target values and ~2.4 to get the 3 target values. I'm telling you it is a beast of a spell.