How do you know if the the exhaust system is blocked?

Hello, Missing at high RPM is normally an ignition issue, and yes, prolonged misfires will destroy at catalytic converter. Blockage? Maybe, but that can be tested for. You need a vacuum gauge to connect to manifold vacuum. Then, run the engine at 2,500RPM, constant, and see if you slowly lose vacuum pressure, and see if engine speed begins to decline without a change in throttle pressure. You could also to a backpressure test at the tail-pipe, but you would want to make the tool, rather than buy it. It might be better to let a shop handle at least the diagnostic portion, but let me know what other questions you have, or if I failed to address your issue.