You'd never know if there was a difference in performance unless you had it on a dyno both ways. Probably just a preference or a psych job. Might let others hear you better coming up beside, IDK.
Since we are guessing about the performance aspect. The cooler header temps may increase the exhaust scavenging effect and also lower engine temps. Didn't see that he ran the headers that way last night.
You want a hot header tube so you have a smoother transition for exhaust gases to pass through.
It's to keep from smashing the tubes closed on a high curb race track like at Perris.