I have ceramic coated headers and side pipes on my Cobra. I've been really happy with them, but not so much for temp reduction, but for good looks and protection from corrosion. The biggest heat reduction I've had was provided when I changed out the side pipe mufflers. The original mufflers had very restricted flows and new ones didn't. The heat radiating from the side pipes dropped dramatically with this change. So muffler choice maybe a bigger factor in heat radiation than coatings?
On another of my cars, replica Ferrari 250 GTO, I decided to use a different coating for the headers and head pipes:
White Lightning. It's much thicker than typical ceramic coating and they claim much better at keeping heat in the pipes. I went this route as the exhaust is under car and very tight to footboxes, tunnel and transaxle. I don't have any comparison numbers to share but I thought I'd let you know about another alternative. In this case, the exhaust is all 304 stainless so I didn't do the coating for corrosion protection, it was all for heat control.
I just got the car back from paint shop last week...
...so I can't tell you from driving experience yet if I've got the exhaust heat under control. Hopefully in a couple of months I'll be able to talk from on the road experience
