hot exhaust = fast exhaust. cold exhaust = slow exhaust. more velocity = better scavenging.
Not even addressing the effects of exhaust heat and turbo efficiency.
typical heat wrap can retain some 50% of the heat in the pipe, faster exhaust,better scavenging. More out = more in.
typical ceramic coatings can retain 25-40%
mercedes benz formula 1 engineers went with a smaller diameter exhaust even though it more restrictive, deciding the higher velocity of the exhaust would loose less heat and offset the hp loss compared to larger diameter slower exhaust.
But like I said some builds won't accept it. With higher end headers very "tuned" pipes built for specific engines not the general "one pipe fits all" pipe. Speeding up the gases beyond the builders design for a particular wave resonance could be detrimental.
As an example the R&B lsr pipe on my 100hp 80" evo made less power when wraped, almost a full 10 by changing the wave resonace. The pipe didnt like it. kinda looked cool though.
In my case even at 1428 and higher compression the gain from heat retention would probably only be single digit mostly due to exhaust length. But like i said 2 from velocity, 2-3 from retained chamber heat from chamber coating, 1 or 2 from proper cam timing In the end maybe only 10 hp...about the same from a stage 7 jet kit and a kerker. at a fraction of the cost. I'll take it.
added is an excerpt from Engine building basics . and some good reading.