Quick testing with the two level 20 copies, it's noticeably better. The copy with Ancient Magic in turn of Trading can kill a Devilkin in five damage pulses and four burn ticks (pulse, tick, pulse, tick, pulse, tick, pulse, tick, pulse -> dead). I think I'll run it from level 1 and see how it fares.
Update 1: I'm running Devout Offense Focus/Concentration/Blessed Defense Focus with Holy Fire/Prayer and all points into Vitality. I did the Gahanka path and all went well apart from some elites which were stronger, but as I outgrew them in level it became easy. Gahanka on the other hand even at level 4 and my toon at level 7 was a tough and long fight. Lots of health potions and I had to bash him repeatedly with a yellow grade pole arm to bring him down. Wasn't easy though, but the rest of his entourage at level 4 died easily with HF and the ocasional Zeal. So regular mobs are in check but elite/bosses are a tougher bone to chew until I get my skills up. Continuing.
Update 2: Ran the toon up to level 20 and until the Orcish Byway, and though I haven't tried going against Gar I think I won't be very good against him. Just a gut feeling. I've gone overboard with the skills and have chosen Devout Offense Focus/Concentration/Blessed Defense Focus/Ancient Magic/Riding/Trading for trying to maximize the auras and shop for +All Skills, which is where I'm at now. I'm completely broke though, so I guess it's probably time to do the Unique Mounts quest and grab the tiger when I sum up the gold for it. Then quest in Tyr Lysia in Normal for gold/gear/+All Skills and see where the build goes from there. If from then on I can follow the main quest and reach Hard I'll be very pleased with myself.
So far Holy Fire has been very well balanced in my honest opinion. It's effective enough that I can run circles around mobs and bring down most of the regular ones, but at the same time it isn't causing that much damage per hit nor is it being a game changer against elites or bosses (the Raging Boar also didn't feel the burn much). I'd say that it's just about right as it is, now I'll have to see how it progresses the more the build progresses as well. But I'm liking it so far.