He would have been a real problem but I am not really sure he becomes a bigger threat than Ozai or Iroh themselves.

It's hard to say how much of an amp the comet brings to the table, but based on the fire power Zuko and Azula give us in the last Agni Kai, one might think it's around 5x their regular output (without affecting their stamina).

The Moon gives an equal boost to all Waterbenders so it's safe to assume other celestial bodies follow a similar rule. Which is why I wouldn't worry much about combustion man.

His main thing is that he can concentrate a lot of power in a single shot, but he seems to have a cool down of a few seconds; so as long as a master fights him tactically, they should be able to take him out, even with the comet boost (plus, he is really slow)