To be honest, I'm with you on this one - even with a decent mani / exhaust system, increased air flow can only do so much.
We ran some before and after RR tests on my old Seicento Abarth (bearing in mind that this car had a janspeed 4 branch mani, MetalKat sports cat, stainless straight through to a S/S backbox, running bigger injectors, fuel boost, quad core plugs and leads), based on the factory air filter (never cleaned or changed in 6 years) vs a PiperCross ring vs a modified ford focus induction kit with cold air feed.
The results were quite interesting - AFAICR, was running 69bhp on stock, 71 on the pipercross and 74 with the induction kit...
I guess it's kinda like the fire triangle thing - in the sense that you need equally increased ratios of fuel air and spark to create the right balance of increase, and as you say Nissan will have spent a lot of money going for a happy reliable balance of power and economy...