I have a good friend with an '07 SS supercharged Cobalt, the Stage II upgrade on the supercharged Cobalts is only about $700, which includes the parts and service.
Which is not bad price for at least a 30hp bump over stock.
He also has the optional oem limited slip and traded someone for a set of 06 Recaro seats.
Inejen intake, DC front strut brace, and sitting on a set Eibach springs- the car has surprised quite a few people in our local NASA autocross group.
I have also driven a Mazdaspeed3, in comparison they are on par for acceleration but the Mazda has the edge on handling and braking.
That said the Cobalt is way cheaper to buy into.