I don't know, the cost seems extreme. Then again, coupled with a recovery power, it will be extremely powerful to the point of being broken. +1 stress per consequence is a nice edge, but I would not go for much more. And I'd probably be careful to do it for physical stress as well. That's what the toughness powers are for.
If you want to grant more powerful consequences to more powerful characters, I'd rather grant more of them than upgrading the existing ones. Maybe an additional mild one for every 2 refresh above 10, a medium consequence for every 4 shifts, severe for 6 and extreme for 8. That would give you more survivability, while being granular enough to not be a waste too often. Also, it will come with more consequence aspects, that can be used in the conflict, which is a double etched blade, but yeah for those. Maybe with the caveat, that you can only use one of each type of consequence per attack, so you don't just soak up everything in minor consequences and shake them off after the fight.