How bad could my emphysema be if it only affects the top part of my lungs?

Look at your PFT. The lung function test or pft is the more relevant measure of the degree of damage caused by emphysema. Two numbers to look at: fev1 and dlco. The lower these numbers, the worse the condition.
Lung function test. The best way to determine the severity of your emphysema is do to a pulmonary function test (lung function test). That will determine how much function, if any, you have lost. Many times the emphysema is the worst in the top of the lungs is because that is where most of the smoke goes when you smoke cigarettes. So, the best thing you can do is to stop smoking.