Read the article this arvo, I don't believe the cars initial tune made the most of the upgraded components, not saying it wasn't tuned tho.
Oddly tho they said they put the stock gtr exhaust, I was always under the assumption that this was quite restrictive for 300 odd Kw application, and often one of the first mods worthwhile on any car
They also said 'peak power' barely change, but mods like cams aren't always aimed at peak power, often mid range or low end improvement is what people are after.
But overall I would agree that manifolds on the GTR only need replacing in high HP setups, often after more crucial components have been sorted first.