Several things
If there is any significant HP increase from using ceramic exhaust coating, I believe it is more from turbo charger installations, which benefit from keeping heat inside the exhaust system, not for normally aspirated engines.
If there is any HP increase to be had in an Atmo engine, I guess it's keeping the engine compartment cool, which helps keep the fuel density higher (since the induction tubes are close to the exhaust). Cool air is denser air.
Also ceramic coatings may have less friction allowing less exhaust backpressure? However I agree the main goal is just to keep the temp down which has obvious benefits on wear and tear on the components under the cowl. I think it is an accept fact that ceramics will lower the radiant heat off the pipes. Is it worth it? All I know my old RV-4 would just COOK the top of the cowl after a flight, and I have seen other RV's with scorched or bubbled cowl paint from engine / exhaust heat.
As far as pipe longevity, leaving them alone is likely the best thing to do. We know that works. There are 30-year-old stainless steel pipes out there. Vetterman may have no interest in recommending it, since his product works without the coating, and there may be some down side yet determined. It is a fairly new thing for aircraft exhaust (no ceramic coated pipes with 30 years service around). If the ceraminc coating is not done well there is a chance of creating more problems than solving.
Welding heat sinks on the pipes, as was suggested, may not be a great idea since it might make hot spots and thermal stresses. Besides, you are still heating the engine compartment and the worst spot to have a heat sink is next to the induction tubes. Cracks tend to happen around welds. Again leaving the pipes alone is best from a "pure goodness to the pipe" standpoint. 321 stainless can get red hot, over and over for years with out degrading. They (the pipes) don't need cooling, it is the under cowl area that needs to be cooled. The RV's have very tight cowls and everything is very close to everything else.
From what I heard if you should get ceramic coating, get it inside and out coated and to the higher temp rating. G