I may have it somewhere, but there was someone from the southern hemisphere (not good to confuse aussie's and kiwi's..
) that flow tested the VH stock manifold, I believe it had enough flow for about 600 HP or so, and that the heads and the manifolds are well paired together.
The primary reason the cayenne intakes are popular are for the 240sx and 300zx guys that swap VH's into them so they can close the hood without modification. I've not seen any documented evidence on an N/A application they do anything.. Comparing the VH's intake to other intake manfolds of engines that make more HP, seems really overkill just looking at it purely aesthetically..