Header Wrap

Fluid flow is the result of an energy gradient; the greater the gradient, the greater the flow. The only time you'd want to insulate the exhaust would be when there's a turbocharger downstream, so that the energy drop across the turbine wheel would be maximized.

The only possible excuse...that I can come up with...for these claimed torque increases would be an effect on resonance, due to the higher temperatures. I suppose this could happen, but I'm from Missouri. As far as I know, this "trick" isn't being used with any of the very high specific output engines.

My theory is that those claims may be based on thruth, if you're lucky, like with many miracle solutions(!).
I suppose that if you do not cool the gases on the way to the next join, they have more volume (more heat is more volume). Thus the timing and duration of a gas pulse that aids extraction of another cylinder at the next join will vary because of this, to start with it has to start sooner and it should take slightly longer, both because there is more gas volume. If you're lucky this may get your 4 into 1, 2 into 2 into 2 etc. headers just into a better mode, on the other hand it may also make it worse.

The turbo advantage I can agree with, IF the turbo did need more gases and could get rid of them. Many systems have the by-pass (partially) opened for pressure control and having more gases is not going to do anything positive for those circumstances. In theory back pressure would increase and actually lower the turbo capacity if the bypass didn't close a bit to compensate. In general I believe the overall result will be more back pressure, and will be counter productive, anyway, it would only benefit a certain limited region of operation. In practice strange things may happen though, not in line with what one would expect, thus looking for the last horses should probably include a test if you have the time and money.

On the other hand, if the wrap doesn't harm horsepower, it should lower engine bay temperature and that can never be bad, especially for access, at a race track when you have to make quick changes on hot engines!

Just an idea, interesting topic, maybe some reputable header makers can comment.