Better spool and power holding
to a point, but unless you are going for a large port head, it could be perceived as pointless, as spending all that cash on the uprated manifold will make very little difference on the actual road Vs a standard one
yes, it may give 10bhp more on paper, but I could not say a manifold for £500 or more could be justified for those gains though, unless going big power
remember it does not matter how bigger manifold you go, as the small port heads can only flow so much, so still you are restricted and end up with a bottle neck type effect, therefore making the larger manifold upgrade pointless
If you were going BT, then a larger manifold coupled with a large port head would make sense