The only problem I see with Magical Self-Enhancement is that a lot of the powers folks will take cost an even amount of points (unless they get the top version of that power), but they get a rebate of 1/3rd the cost of the power: if nothing else, you should establish whether players round up or down.
My suggestion:
Most of the scalable powers (Toughness, Speed, etc.) tap out at 6 points. 1/3 of 6 is 2. So let's set the +X for this as +2, with the caveat that the power cannot be reduced below 1 Refresh in cost. This may serve to encourage folks to get the 3- to 4-point powers for ultimate min-maxing, though, so that may be an undesired consequence depending on your view. But we could use the pre-existing Catch phraseology.