The exhaust gas in the header will be in the range of 700-1000F and that is more than adequate to fry the elastomer holding the fiber gasket together. But once the elastomer has been effectively "baked out", there should be a dense enough fiber mat left to act as a seal. From this point on, the success of the gasket is the rigidity of the flanges/bolting. The fiber mat will seal if the joint isn't moving with the thermal cycles. Even seemingly negligible movement in the joint will break up the mat and result in a leak. These elastomer bonded fiber gaskets are used successfully in a lot of high temp applications, but only in well designed flanges. And sad but true, most header joints aren't stiff enough to be reliable with the fiber gaskets. Bolts undergo stress relaxation with time at temperature-loosening the joint. And the thin low carbon steel flanges have the same problem. The graphite gaskets will take the temp and are more forgiving of a weak joint-but only to a point. And thicker usually ain't better. It is just one more loosening problem as the mat looses bulk (elastomer bake-out). Check the flanges for flatness since overtightening weak flanges to make a seal can warp the flange and make an even loading on the gasket impossible. The hard copper gasket will make this especially important since it doesn't have much give. And good luck!
