The reason for the stress cracks is what? Heat cycling I guess.(I'm not sure) The reason for the coating is to keep the heat away from the header (or at least inside the tubes) The exhaust keeps the heat and not the headers in theory. So I would think that this coated header will outlast the uncoated part because it keeps it cooler. This is the selling point that coating companies sell. I used to work for one.
1977 911s. it's cool