The internal shape of the stock throttle body was highly tuned from the factory. Since there is no idle air control motor the drive by wire computer moves the throtle blade slightly to control idle speed and decell. Any changes in the TB will change idel and drivability. There is also pleanty of air flow so the gains will be small
If you want one though, SaturnMotorSports sells one for the Redline that they just released. No reason it shouldn't fit the Cobalt as well. You can email them. It's not a huge power gain, for the above mentioned reasons. The claim a few horsepower, but state that throttle response is much more crisp and precise. They will also tell you that it's going to be more useful one you have an intake, header and new exhuast setup.