Yes, I don't use RTV (even high temp) anywhere near an exhaust header. RTV High Temperature is only good for 700 degrees Fahrenheit. Temps there can far exceed that. The ceramic header coatings are good for 2500 degrees and stay put. The problem is not just heat burning them away, but the fact the header flanges move on the head during operation - just like heads move on the block during operation. Aftermarket tube headers really do need a gasket. Cometic has an .030 stainless steel gasket that might work for you. There are also some .040 copper gaskets. You do know it is alright to reshape and ding headers for clearance where needed? The jigs used when building them aren't always perfect. Even baked on coatings can distort them some. I have a 1970 1/4 mile nitro methane drag car, I built new '71, that has special big tube crossover headers that I really had to work hard on just to install them - even down to reshaping the underbody and steering drag links for clearance. Before I worked on them, there was zero clearance anywhere - and they were custom built for the car on a jig by a manufacturer. Other cars have also pretty much always required some tweaks for any headers I used on them - some more than others. I had to notch the frame rails and rework the transmission shift rod linkage on my automatic '81 GMC to gain clearance for its stainless steel headers. These are just things you run into with aftermarket headers - even very expensive ones "designed for the vehicle".