The ole "brittle" argument. Commonly used against BST wheels and not true there either.I don't know about burning the valves, but I do know it will make the titanium brittle over time from trapping the heat in. Believe me when I say that exhaust wrap is a fad with no top race teams using them. I've seen it done on some Ducatis but more to avoid burning your leg than anything.
Hotter = less dense. As exhaust gas cools, it expands which slows the flow.Wrapping keeps heat of hot exhaust gas, in the pipe(s), and that is a good thing to reduce heat to area outside the engine, getting back to the engine.
As far as performance on a naturally aspired engine... It ain't helping... Thingy is.. Hotter exhaust gas, is larger volume, and that is a tad harder to get out of the pipes.
Now if it is a turbo Charged engine, the larger volume and higher speed of the gas getting pushed out, increases the turbo speed, and that increases the boost.. So that is a good thing. :thumbsup: