Our Rusty would sometimes cough and lay on his belly with his legs sticking out all over with his mouth hanging open after tearing around. I took him in to get checked over when he got his shots and the vet said his heart sounded good but to watch him.
That was a few months ago, he is about 1 year old now and getting to be a big kitty but I have not seen him have a hard time getting his breath in a long time now. Maybe now that he is older he is a little more calm, but he still tears around like he always did. We also went from cheap cat food to Nutro indoor, maybe that has helped in some way.
He is a nice kitty who is kinda shy but get him near some catnip and he is totaly the oposite. Any other cats come near his catnip and he runs them off so its all for him lol.