Zeref almost killing Aldoron doesnt scale to anything. It only happened because Wolfen thinks Aldoron is precious, and Ankhseram's curse triggered. The only way Zeref can fire off his curse against Aldoron is if he started caring about life again, and when that happens Zered becomes so weak that Ultear can beat him up. Zeref and Mavis killed each other with the curse when Zered was beaten up and couldnt move. So, all in all, the curse isnt an indicator of combat prowess in the first place.
Ignia's power gave Natsu a more substantial powerup. He wiped Mercphobia's MP and badly burned his own arm despite his slayer resistance to fire. The last time he did something like this was against Zeref with his flames of emotion. Though since this is an FDK spell, this can simply be how hot Igneel's flames can get with sufficient magic power.
But the tiebreaker is how Ignia is treating the other god dragons. He called Mercphobia a weakling, and made Selene look easy. Selene mades a big deal about how god dragons will wound each other, but Ignia seemed willing to just talk and face-tank.