from the evidence presented over the years from cars like Christians, Tims and Ollies i'd forget changing the standard manifold. there are better things to spend money on from what i can see. the only time i'd bother changing the manifold is if i wanted to move the position of the turbo for packaging reasons. as i'll never get to the stage where maxing the manifold out will be an issue i'll personally be keeping the standard manifold and spending my pennies elsewhere like on getting the shell spotless and rust free.
regards
Tom