from my reading it is that you raise your caster level and are stunned for that many rounds. That increase is limited to up to +3. However when using the feat it removes the cap on the spell entirely.
12th level wizard casts fireball: it deals 10d6
12th level wizard casts fireball while increasing his CL by 1 with this feat: it deals 13d6 and he is stunned for 1 round.
That is what i see when i read it. Good thing it said "increase cl by 1,2, or 3" because if it said "increase by up to 3" i would use it on every spell ever for +0 cl to uncap the dice.
I was going to ask why you couldn't do it, then I saw your point.
Still, if you can't be stunned, you only take 1d6 damage from raising it by one, it's an awesome deal.
EDIT: Also, remember myself to tuck in Tainted Scholar.