The reasoning behind using SM and making spells cost more PP and attain higher lvl is that it IS the way spells work. There is a base concept: say detecting stuff, and the first lvl spell on it is about detecting something minor. Higher lvl spells give more important data for higher PP cost.
With SM it is the same way: you learn it for one list only, more power in the spell costs more power points and then you need to prepare more, have a really pimped out SM skill and you need to use more power points.
Using SM just to change the color of the spell or have a fire ball just light some birthday cake candles might not cost more PP, but in a way it is pretty impressive to bend an 8th lvl base list spell to doing something minor as lighting tiny candles AND NOTHING ELSE in the room (stress placed purposefully since that is the important bit of the spell mastery.)
But anyway: if you don't like it, either don't allow it, or don't develop it with your mage, but I'm convinced someone will like this rewrite of the SM skill. It is very much a way to take the spell list concept as a whole to a new lvl. Isn't every spell list perhaps a base spell, with other spells developed from that one spell placed on the higher levels of the spell list?
As I wrote earlier: me and my nephew looked at plenty of lists to verify his spell mastery system to see if it worked out, and to us it made sense. ToM even increased some costs in PP, to balance it out even more, so yes: it might not be that super powerful skill it once was, (especially in RM2) but its still workable. Nerfed, but not to death.
@ToM: Table looks great! Hope people here like it.