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.
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Cooler gases have less volume, so they pass the exhaust system easier.
Hotter gases requiere larger diameter and cause increased back pressure.
Simply physics.
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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.