Clarifying Vox's powers because there is confusion
So Vox didn't become the most power sinner in hell at that moment because of Charlie's word. Otherwise, we would see Al chain break right after she says it. It became true after Vox's approval rate hit 100% and we see him transform (afterwards we see Al's chain break). I understand the confusion the show doesn't straight say it, though it does show us that Vox's raw power increases with the amount of influence he has over the people, which make sense because that is how he farmed for power in real life as tv host and a cult leader.
Ep2 of season 1 also showed us this (though I thought his power increased base on the number of people he could hypnotized). On top of that he could still gain power through soul deals. So, some sinners can manifest in hell with special abilities to help them climb the ladder faster, though they are probably rare.
Another thing to clarify is that Alastor made this calculation to free himself his deal with Rosie because he was in a weaken state. He knew Vox could surpass him while he was nerfed. Al in their flashed black said that Vox was the sinner that could get the closest to his power level, and it's because of his special ability.
I don't think Al got a power boost when his deals where broken. I think he was always that strong, but he wanted the Vees to captured him in ep 4 and he wasn't going all out. Cause remember his deal with Rosie didn't break until the final ep. Which means he was stronger than each of the Vees individually in that fight (I don't know about all together).
And yes, Vox was more powerful than Al in their final fight, in turns of raw power and strength, Vox had Al on the back foot for the first part of the fight. Al changed his strategy and tactics to gain the upper hand in their 1v1.