I know I'm not a pro and may be talking out of turn but!
Quite frankly if your are faced with an actual fire emergency the difference between flying 180 or 240 minutes is irrelevant...
That would be fine if the problem was positively identified as minor. But I'm not sure an extra hour is irrelavent if you can't identify the issue. A fire emergency may not be an actual fire but just the smell of one.
Isn't it possible that you could have an issue that calms down after you unload all the non essential buses etc, but is still slowly degrading unoticed? Then isn't it possible that it could rapidly escalate, leaving you with the feeling that, maybe you wish you had taken the shorter option.
As always its up to the flight crew to determine whats safe in their particular case.