Balancing the IoPs powers using the stunt/power creation rules is your best bet. For Fate expenditure stunts, the 'standard' is a Fate point for 3 shifts of effect, once per scene. So it would be hard to argue that the following power is unreasonable, especially since it is identical to the 'Killer Fists' stunt:
Blessing of Heimdallr [-1]. Add 3 to the damage of a successful hit with the Sword of Heimdallr, once per scene, for a fate point. This stacks with any other damage-increasing stunts or powers.
Note, by the way, that since the base weapon has a weapon rating of 3, this works out to being identical to:
Blessing of Heimdallr [-1]. On a successful hit with the Sword of Heimdallr, once per scene, spend a fate point to make the Sword cound as having a weapon rating of 6.
To be honest, I think that either of these is somewhat on the weak side, since they grant in effect 1 shift of benefit once per scene, and only when a fate point is spent (since a fate point is generally worth 2 shifts on its own). I think I'd be tempted to boost it along these lines:
Blessing of Heimdallr [-1]. Any time you invoke the aspect linked to the Sword of Heimdallr to aid an attack using the Sword, you get +3 to the roll instead of the normal +2. If you choose to invoke for a reroll, you may "lock down" one of the dice, rolling only the other three OR you may reroll all the dice and add +1 to the result.
This last one is a bit like a inverted version of the 'Unshakeable' stunt.
I hope some of this is useful to you!