A few thoughts:
I have Jet-Hot coated several exhaust systems and been satisfied with the results. Jet-Hot can coat both the inside and the outside of the headers and exhaust which I typically do. On one car, I measured the surface temps of the headers about 6" from the cylinder heads before and after and noted a reduction of 75F in the surface temp. Also, I have not had any issues with wear and found the coating to be durable. My vintage 911 system is 18 years old and still looks great despite over 65 autox events and a few track days. Another system is just over 10 years old and even the exposed side pipes still look almost new. The silver Jet-Hot coating on my 26 year old 7 still looks great.
In the engine compartment, it is also a good idea to wrap the headers with a good heat insulation (I use the titanium heat wrap from DEI) as that can lower the surface temps another 100F-120F.