I just spoke to HPC and they said their standard silver coating is good for 700 degrees. He said something like a 6.0L wouldn't get anywhere near that temp. The only time he's seen a bit of discolouration (not flaking) is on some of the Windsors where the header pipes have very tight bends close together. They have a 12 month warranty but he said if flaking or cracking occurs within 5 years they will redo the coating. This involves a caustic dip to remove the old material first. I guess if that happens you could upgrade to their extreme coating.
http://www.hpcoatings.com.au/pages/hiperext.htm