A problem with ars diabolos is that flame wave is generally better than apocalypse but you're getting a +2/+7 shield for it that emphasizes your worse of the two skills, but because of that base +25% its often bigger than you could get on a good magic or rare tome anyway. If you could get a +5/5 book with 2x facets it still might not be better because its 2/7 skills and +30/-5 fire, versus +5/5 skills and +10/-10 fire, and the rare would be insanely hard to obtain.
So ars diabolos is still the best option, just because there aren't other better ones. And its going to be better than a +20/-20 shield, which is better than a phoenix.
another reason why there's a huge gear overlap between blood boil & fire warlocks. Mang song + enigma + (2/20/2 circlet or hellwardens cheaper) + 2/15 ammy + arachs + magefist + 2x opalvein + (rare boots or wraithstep)
is identical in both builds, only differences being ars baalos vs ars diabolos and demon skillers vs chaos skillers. And you can get away with a +2 demon/fcr crafted ammy for blood boil, but on a fire build you really want +2 warlock so you get both trees.