If CB's durability level is not that high how could he have gone up against the Phoenix Force Juggernaut and Thor, The only way i can see HT's flames affect him is due to CB's powers because they now work on the level of his own confidence and willpower, plus with the nano mist affecting everyone and yet Brian is wearing his old outfit that gives him his powers so he should be as strong as he was before he got his power up and so any of his confidence and willpower that he has should of added to his powers.
The upgrade i was talking about is the one CB got during the Secret Invasion were Merlin made him out of magic it's just that at the moment with his powers Marvel hasn't put a limit to them, So this is just an idea but he might be a bit like the Hulk you know with no limit at all and have you seen CB's respect thread on this site yet some stuff are missing like the new stuff and sorry i don't have any scans of them.