Can having scheuermann's kyphosis of thoracic interfere with lung function/breathing?

Extremely rare. When kyphosis angle exceeds 100 degrees (rare), the sharply bent spine puts pressure on the heart, lungs, and intestines. When this occurs, patients may tire quickly, suffer shortness of breath, feel chest pain, and lose their appetite.
Not typically. There are some spinal deformities that can affect lung function, scoliosis being the most described. It has been pretty well described that scheuermanns kyphosis does not typically severely affect lung function, and so that alone is not used as a reason to do surgery for that condition. Thank you for your question.