Hotter gases are easier to force through the exhaust system than cool gases due to density. Ceramic coating keeps gasses hotter within the exhaust system helping to decrease exhaust density & increases exhaust flow.
The opposite is true.
Cooler gases have less volume, so they pass the exhaust system easier.
Hotter gases requiere larger diameter and cause increased back pressure.
Simply physics.
In the end only one aspect is really significient: taping the downpipe will
reduce thermal radiation to the engine bay and intake system and increase
power on prolongued full throttle.