AFAIK you are correct with the induction kit sucking in more air, thus improving power on most cars.
The 350z is a little different though as the ECU is quite restrictive with performance mods. Say you added an induction kit, any extra power will be taken away by the ECU re-restricting the air flow.
With Exhausts the larger the bore the less restriction of airflow again but the 350z needs back pressure so you will lose torque.
So as martin says, the only real differences you will find from changing the Exhaust and air filter etc are the sound and the look (bling factor)
Hope this helps a little. (also hope im not talking out of my backside too much )