Nulon dot 4 boiling point is over 280 deg Celsius according to their website. From a bit of research this appears to be way more than the radiant heat of headers. Let alone what it would be once getting through the hard line. Some numbers here on pacemaker website. It's advertising numbers, but it gives an idea.
Pacemaker Headers Australia - Quality Performance Exhaust Headers :: Ceramic Coating
Your headers are coated so I think it will be fine.