You did not indicate how many miles you have on the car. Certainly if your mileage is not excessive, it should still be under warranty and there is no way on earth you should have to pay for the air filter and labor (especially the labor) Go back to the dealership, speak to the service department manager, and get an explanation why this is not covered under warranty. Tell him/her that if is not covered you want a letter saying so that you can contact BM (look in your manual for list of contact phone numbers) and report him. Also, you are NOT restricted to the dealership where you purchased the car. Go to another Chevrolet dealer, explain the situation and have them look at the problem. There are so many possibilities that could cause the problem of not going over 55. But it sure sounds like a software issue related to your cruise control that somehow locks you into that maximum speed.