Thanks for the post, but what difference would "internal rotting" make with or without the "cool collar". Where is this internal "rotting" happening and why?
Giving this some thought myself, I now have more questions. It seems to me the weakest part of a header design is the weld and not the unmodified portion of a stainless tube. To avoid getting water inside the exhaust tube if a weld leaked, they moved the water jacket weld away from the flange. This way if there was a leak on that water jacket weld, it would leak OUTSIDE and not go into the engine via the exhaust port.
So if it doesn't leak there at the weld, it has to be internal. Is the inside pipe getting too hot due to lack of water flow? Is the inside pipe not centered within the outer water jacket pipe? How is the inner pipe kept floating in the center of the water jacket pipe?
These pictures are of the "cool callar" type. To me the rusty one is an external weld leak and would not have caused water to get in the engine causing a failure.
Talking out loud here!