Umm the only fire leads that heat proof is useful are 1. Shuca Tran 2. Sash Ape and you lose to both even with heat proof. =/ Azelf too but even without Heat Proof you beat most of them anyway when ev'd correctly.
Bronzongs NEVER use Iron Ball, EVER. Check the smogon usage statistics if you dont believe me. Next its really not a surprize, i for one always make it my priority to Earthquake a Bronzong if i see it taking reduced damage from fire attacks too.
Bronzongs biggest asset is being able to switch into Earthquake, without that its just another asset it doesnt have over Metagross and Jirachi.
You say that he loses to both, where's your proof? Looking at the damage calculations, you;d be suprised to see how well he can fair against Heatran, Infernape, and Azelf. Also the most common other leads are Metagross, who would assume you have Levitate and wouldn't Earthquake, Swampert, who would do the same as Metagross usually, Tyranitar to star Sandstorm, but it doesnt affect the damage calculations and it wouldn't use Earthquake, and it fairs better against the rarely seen Tyraniboah, because they usually carry Flamethrower.
Standard Bronzong with Heatproof against Naive Azelf's Flamethrower: (22.49% - 26.63%) 5HKO-6HKO, with Leftovers calculated in it fluctuates
Standard Bronzong with Levitate against Naive Azelfs Flamethrower: (44.38% - 52.66%) 3HKO with Leftovers
Naive being the standard lead nature, with Heatproof the Bronzong has a much greater chance of being able to stop the Azelf, if it goes straight for Flamethrower it has 4 turns to Hypnosis, Gyro Ball, Explode, or set up screens depending on the set. If it takes out Azelf, it can be used later for other walling, and if you have a bulky Rotom form or Zapdos, Gyarados, anyone to take ground, you're usually set. And people may be apprehensive about using Earthquake, factoring in your ability to neutralize it with the preasuumed Levitate.
Standard Bronzong with Heatproof against Standard Heatran Lead Fire Blast:
(47.93%-56.80%) 2HKO, possible 3HKO with leftovers
Standard Bronzong with Levitate against Standard Heatran Lead Fire Blast: (94.67%-111.83%) OHKO with 33% chance of 2HKO
With Heatproof the Bronzong has a chance to Earthquake the Heatran, and yes there is a Shuca berry usually, but the EQ is not to do much damage, its to get rid of that berry's effect for a Tyranitar or Metagross to OHKO Heatran later, as well as the Hypnosis chance of sleep. This is also assuming Fire Blast hits, while with its accuracy anything could happen.
Standard Bronzong with Heatproof against Standard Infernape Lead Fire Blast: (37.28%-44.38%) 3HKO with chance of 4HKO with leftovers
Standard Bronzong with Levitate against Standard Infernape Lead Fire Blast: (73.37%-86.98%) 2HKO
All of the damage calculations are the same with Fake Out, which doesnt do anything with leftovers. Considering Infernapes using Fire Blast may potentially miss, and that Earthquake will ruin a Focus sash, it can generally hurt Infernape if it trys to stay in.
So Heatproof Bronzong can potentially screw up many of the most used leads in the current OU metagame, easily handling many of the common leads.