Yeah, we should start our own thread on this. What I've written there was from our gasdynamics lecture, and the prof had as an example a double wall header with him, can't remember what off, bmw, merc or porsche. It only makes a tiny improvement, and it's just at that limit where the additional cost might not make it worth.
the gas behind the sound/shock wave (will be a shock wave with maybe only Mach 1.001) is what matters, the hotter it is, the higher the pressure, since the sound/shock wave won't be influenced much by wall temperature (well the boundary layer is there as well, it can increase or decrease for cold walls, can't remember however what that parameter was). When the sound wave (etc. the contact surface) exits from the exhaust, the pressure difference will define the strength of the rarefaction wave travelling back (rarefaction waves always travel at the speed of sound), hence the hotter the gas remains, the stronger the "suction" effect.
I agree that the most common reason for double walled exhausts on motorcycles is to protect the chrome.
Titanium is nice, but expensive. I wonder if you could make them out of aluminium alloy as well.