Hmm, I'm not sure what to make of the scan. On one hand it does appear that Martian Manhunter is weak against flame, but on the other, it seems as though the sheer power of the Daemonite's attack is what causes him to pass out. I can't comment beyond that, simply because I don't know the context of the scan, nor know much about the general Daemonite's power level.
As I mentioned in my earlier comment, that one panel of Martian Manhunter in JL had him apparently fighting the entire League. Of course, as you said, we don't know how that battle turned out, so it's not an entirely accurate depiction. I will say though, that the panel's implication seems to be that J'onn is quite powerful. Batman's comment that 'it ended very badly' seems to support this assumption.
I haven't read Stormwatch, so I don't actually know much about MM's capabilities. But I recall Johns stating in an interview that he wants to use JLA to make MM one of the 'most dangerous characters in the DCU', so I'm hoping that he really does shine as a powerhouse.