Judgement is the only good looking set we've had (Lightbringer is okay I suppose). Once in a while its worth just throwing it and my Ashkandi on and RP walking around IF like the good old days. That is really off topic though.
As as much as you argue the efficiency argument, being at 89% mana when everyone else goes OOM doesn't help any when your tank is dead. Efficiency matters on paper only, in the real world unless you seriously outgear an encounter you can chain-chug to make up almost any mana loss. I'll bite though.
I'll say it again, HL builds might not provide the pure efficiency of FoL spambots, but they make up for it in flexability. Light's Grace is always up so you can react quickly with your DF + HL XI macro, if someone is 2.5k down you don't have to use FoL twice to top them off (which incidentally results in lowered effective HPS) but just 1 Rank V or VI, etcetera.
You asked for math, so here is a little tidbit. This is pure theorycraft, so of course everything is ideal.
Assuming 2000 healing, no set bonuses other than 6% HL crit, typical 42/19/0 spec, BoL on target, 29% paper doll holy crit.
In 100 casts your FoL will crit 29 times.
Flash of Light VIIAverage: 1710
Average crit: 2565
Mana from Illumination: 3132
Mp5 from Illumination: 104.4
Total Healed: 195795
Total HPS: 1305.3
100 casts for Holy Light V with 40 crits.
Holy Light VAverage: 1919
Average crit: 2878
Mana from Illumination: 6600
Mp5 from Illumination: 165
Total Healed: 230260
Total HPS: 1151.3
So now we can look at costs. We're going to look at the base cost of the spell (I will be including
[Libram of Absolute Truth] for HL costs because yes, its that important to the build), find the effective Mp5 lost, factor in Mp5 gained from Illumination, then compare the ratio of HPS to Mp5.
Still using the same factors, and still 100 casts.
Flash of Light VIICost: 180
Total mana spent: 18000
Mp5 spent: 600
Mp5 from llumination: 104.4
Effective Mp5 loss: 495.6
Holy Light VCost: 248
Total mana spent: 24800
Mp5 spent: 620
Mp5 from Illumination: 165
Effective Mp5 loss: 455
Look at the HPS:Cost ratios for the two spells.
FoL VII: 1305.3 : 495.6 ~ 2.634
HL V: 1151.3 : 445 ~ 2.587
You are not losing much by downranking. Not anywhere near as much as you seem to think.
I'll do more later.