I like this and have been thinking of a very similar approach, but I fear it might slow down play too much, requiring the extra roll. In my mind, I would prefer to roll once for the spell. If the roll fails, the misfire is determined by the existing pool somehow.
Current thinking is that the highest die in the pool determines the severity of the misfire. The severities increase with the level of the spell. So you have a misfire table with 1 through 5 as rows (highest die rolled), and the columns are spell levels. Make sense?
I was thinking on a continuation of my corruption-take on the magic system that could work well with this, setting-wise. Let's throw corruption out the window and say that if you push while casting spells and roll the equivalent of bio-hazard from MY0, you accumulate residual magical energies, kind of like fission creates radiation. So you could cast spells as much as you like, but higher levels of magic causes greater accumulation of residual energy which in turn increases the risk of magical disasters.
However, the residual magical energies don't dissipate by themselves, so pushing even small, simple spells contribute to amassing dangerous levels of unstable magic that can blow up in the casters face at any moment. The only difference is that pushing high level spells gets you to critical mass faster. Kind of like the Rot works in MY0, so everytime you gain a new point of magical residue, you have to roll for your total amount of points, where critical failures cause side effects to your spells (maybe some beneficial too? It's wild magic after all, not evil magic) or even full out vortexes of wild magic. And the different levels of spells would work similarly to the different zones of Rot i.e getting a bio-hazard on a level 1 spell nets you a single point of residual energy, while a level 3 bio-hazard nets you three points.
Getting rid of the residual energy would require the Spellbinder to slowly leech it out over time or through some kind of ritual. Maybe some sorcerers have deviced a volatile form of magic that feeds on wild magic? (Demonology, anyone?)
Dangerous enough? =D