With regard to the wording, adding another target doesn't necessarily mean you deal double damage...a cunning DM will HALVE the damage taken by each target since each bolt carries half the power. (Doubling the damage is an additional -20 modifier making the total modifier -70, so with the example stated an average roll would be a 55. Good Luck with that. ).
I don't like it, an additional target is an additional target, halving the effect is complicated. For a single firebolt is simple, but what if I want to add another target to my Teleport III?
All in all just a quick example of how it could be applied, I'm necessarily saying that this should be applied to all uses of the skill, just specific ones. In the case of the Fire bolt, I'd say that the Magician would either have to split his Directed Spells OB or half the damage..
With regard to Teleport III, well that, to mind is a cut and dried example, not subject to a great deal of subjective reasoning on the GM's part. It's not a great game-breaking use of the skill.
That means that a level 5 mage shouldn't go around spellmastering spells he learned just yesterday... Spell Mastery is for experts!
Seriously, SM isn't that difficult, provided that you don't roll a failure, even if you score a Partial Succes means that you have succeded in 20% of your action, you just have to keep rolling over 75 until you reach 100%...
Apart from the 10 concussion hits per attempt that it is...
Fair enough. Strictly speaking a 5th level Magician can't cast a Firebolt unless you allow overcasting (so moot point..
) and that incurs even greater penalties..
I chose 5th level simply because a Magician gets the skill as everyman, and at 10 ranks that's the most economical bonus/dp ratio. After that the Magician is hit by diminishing returns. A spell-user that doesn't have the skill as everyman needs to be 10th (or spend another 50 DP..
minimum to achieve the same sort of bonus..
The point I am trying to make is that in most cases it is the GM letting the player pull the wool over the eyes of the GM.
In your example,
"I cast Waterwall True, and use spell mastery to give it a round shape around me and make it move with me as I walk through the flames.."
A GM should simply work out the relevent modifiers and apply them, if they aren't clearly defined by the Spell Mastery discrption then make them upon the fly.
Example. Change shape from Wall to 10' Radius -50, allowing it to move -30, caster doesn't need to concentrate to move it -20 = -100.
Now, assume that the magician is now 10th level and has taken the time and effort to concentrate on Spell Mastery (Water Law)...and has 20 Ranks in it, then their bonus is going to be +30 greater than my previous example. So, a grand total of +105. Humm, so an average modified roll is 55 (Fail).
I know that your example was just one pulled out of a hat, but, I quite like my players trying to be inventive and I'm not a great fan of spells a la Canon, so if they come up with an idea then I'm not going to penalise them for being clever and trying to think outside the box. And that is basically exactly what is being suggested by adding an additional PP cost..
and I agree with Rasyr, it has been subject to an element of power creep. The trouble is, make this skill any more restrictive then you end up with a D&D type situation where more and more "ever-so slightly different spells" are printed and are spoon fed to the players (giving the GM's additional headaches of keeping track of them all) rather that letting the players be imaginative with the use of the spells that they have got.