I don't feel that the wizard is underpowered as such, but he ofthen demands specific situations for his spells to really pay off. What I like to do (when I'm GM) is create quests that have a couple of rooms that are aimed towards specific spells. A couple exsamples:
You could have a treasure/whatever room only reachable with pass through rock - remember to put in some kind of one way exit.
A room with scores of low-mind monsters for wizzy to put to sleep.
Room filled with high-end monsters to mist through.
Put in rolling boulder traps which the tempest can help you escape from.
But regarding your suggestion:
I think it should just be a fixed roll... e.g. roll 4+ to keep the spell. Seems like it would be about the same, just with less rolling.
If you keep the roll-off I definitly think you should remove the rule about a tie roll. The player has 9 spells... Having to remember that maybe 3 - 4 or more spells are now one-use-only is too difficult.
edit: But try it out and tell us how it worked out
