I have to agree with PeterR. SM wasn't intended to make "more" out of "given amount." That's been our interpretation across all the GMs we had for RM. There are conventions and specific rules for "Increase duration", "Increase # of targets", etc. and the challenges/penalties can be pretty steep.
Spell Mastery, as we saw it, was to "have the fireball curve around an obstacle to hit the target", or "use the ball or darkness to obscure the light of a moving torch", "use a gust of wind to get the paper on the desk to blow out the window to the waiting person below", "make the fireball pause 3 seconds before exploding." It was just minor tweaks to basic mechanics to make something a little trickier occur.
We had one player who used to use SM to create a giant, "frightening" show of summoning the holy powers of the heavens so scare of hordes of goblins or simple peasant folk. The fiery, scary light show did nothing to enhance the attack spell, but it made those watching it tend to break morale and to flee. The better the SM roll, the more showy and believable it was and therefore the more difficult on the RR to not flee.