Very interesting. I'm thinking about picking this up for my Castles & Crusades games.
Why? C&C has an implied character level cap of around 12th level (6th level magic). Why complicate it?
If you want to add a spellcasting check system make the CL = to (Spell Level x 2) - 1. This results in a game where a spellcaster will succeed 40% of the time in casting spells appropriate for their level.
As compensation for spell failure, treat spell slots as the number of spells that can be safely memorized for each level and the number of castings per day, remove "fire and forget".
Add a spell weave/unweave feature. Weave/unweave states that two spells of one level can be weaved together to make one spell of the next level, with the inverse being true.
Allow over taxing (temporary CON damage) to continue casting spells beyond his daily limit, with CON damage equal to Spell Level.
Allow characters to memorize spells of a level higher than appropriate for their level, with an Int roll, and CL = to (Spell Level x 2) - 1. The result being that a low level mage may very well have a Fireball up his sleeve. He might not be able to cast it (most of the time), and it might kill him to do so. . . .
I think this is the least you could do to balance Magic with Chance of Failure against the default 100% Success, Fire & Forget System of C&C.
Just my 2d.