This made me think of an interesting take on the rage timer...on the other hand, if you really want to wipe an underpowered group - maybe the rage timer should go off after 10 minutes if they haven't made at least 25% progress. IDK
How about the timer scales as the fight progresses? At 10 minutes if you haven't done at least 25% damage then the boss increases rage damage by X amount, at 15 or 20 minutes if you haven't done 50% damage it increases more and continues to scale thus encouraging well formed/balanced parties to tackle these bosses.
This way the boss doesn't just automatically turn into a near unkillable beast once rage kicks in at 15 minutes... it gives groups a fighting chance.
On a side note, is the timer valid for all mob bosses or just the newer quest bosses and aggy?