Your '“k-tuned” headers' link needs touching up.
This all sounds reasonable, 4-2-1 would definitely be the way to go. Except I would think the exhaust port should be matched. A round pipe on an oval port will induce turbulence.
Ceramic coating would be a preservative. It may/may not be more effective for heat than wrapping with a thermal tape.
For after your catalystic converter:
If you look at vascular structures like capillaries, you see structures like this:
http://www.fractal.org/Fractal-Research-and-Products/Fractal-Tissue-Engineering.htm
The 'pipes' don't have a constant cross-section in the vicinity of the branch. At the very least, the branches should be a lower case y not an upper case Y.