It's optimistic going by UK figures but very well documented state side. Right through all the mods we consider the max for bolt ons but as no one in the UK runs long tube headers, or bigger tb, or bigger maf and intake we have never seen it
NWP have dyno plots etc of the difference a larger tb makes, a tb has a butterfly etc inside which disturbs the flow don't forget
The key strangle points on a normally aspirated vq are the manifolds, tb and intake pipe/maf, then the plenum, then the cams etc
The tb, inlet pipe/maf were improved on the hr by doubling up, something nissan didn't just do with no reason to. The hr also had significantly improved manifolds but still compromised due to emission laws needing the cats close to the engine.