One argument for 'any' R-Factor associated with the 'exhaust system' stems from under-hood intake snorkels, where charge density ( horsepower ) suffers from 'ANY' elevated temperature.
Wrapping the headers, downpipe, turbo, etc., reduces the heat transfer coefficient from the exhaust system to the engine bay, providing the lowest possible ambient air temperature entering the intake tract, hence, greatest horsepower.
There'd also be an extremely small increase in available blow-down gas energy available to the turbine side.
-----------------------------------------------------------------------------------------
Perhaps the 'best' reason to do it though has to do with burn protection in the pits, between runs.
