I think I am in the "so what?" camp here. High level wizards should hit enemies hard and often -- Disintegrate is a nice spell but hardly gamebreaking. It can actually fail agianst opponents with SR, high touch ACs and good Fortitude saves. One level higher is Finger of Death. One level lower is the extremely nasty Dominate.
Fighters are going to full attack -- that is not necessarily all that much less effective than Disintegrate at that level.
He is a wizard with a signature spell. That can actually be fun and part of the mythos of the party. Of course, some opponents will know this and take counter-measures.
The only time that it really matters if your encounters are repetitive. I think that some hordes of foes encounters will help as will challenges with Outsiders and monsterous critters (see Dinosaurs; Triceratops is a CR 9 critter with a +19 Fortitude Save).
If the wizard is level 11 (minimum to cast this spell) than a Barbed Devil has SR 23 (60% to block spell without the use of feats) and a FORT save of +14.
And even though he'll small each one easily, consider what sixteen medium Air Elementals could do. The Melee types will tear through them but a blind use of Distingrate will tend to be suboptimal.
Otherwise, I think that this will all work out well.