I have nothing to add about the diameter of piping, but I did have something to add about what you can do to keep the gases as hot as possible.

You can use header wrap the whole length of the exhaust. Then you get the silicone spray for it. This keeps the wrap from rotting out your pipes. This will keep your exhaust as hot as possible.

Just a thought if you're trying to do as much as possible. Another side note......if you do what we were talking about in that other post, you can't use the normal headers.....they don't bolt up.