I have mine jet hot coated, but I didn't do it for "practical" purposes. I just did it to change the color of it. I wanted to have a more polished look.
However, I do think that it keeps the metal of the tube from getting as hot as it would without it. Comparing metal to skin, isn't exactly quite the same, considering completely different thermal properties. But honestly, Bur is right.. the rate the air flows through the tube, the difference "if any" would be so miniscule you probably wouldn't see it, even if you looked in the tenths of a horsepower range.