I'd glow cherry red at 20k toolll
I'm not say'n it ain't gonna happen, and no, I've never done it myself, what I'm eluding to is at that point you are at the limit, either time or material, run it continually at that point and you will limit the life of the engine and/or waste perfectly good parts.
Now with computer controls and advances in metal science engines today can withstand higher temps but keep driving that turbo at the limit and I think you will cut engine life 30-40% or more depending.
Glowing headers in a everyday street car = bad
Glowing headers on a dyno looking for that last couple ponies = yeah,ok
So I'm kinda agreeing with you but not really,
DISCLAIMER: this is an opinion and not based on any real mechanical experience, do not take internally, and do not induce vomiting.