I was thinking...I didn't address options for people that don't want to put on a super charger (S/C's are expensive and complicated to install).
From my perspective....more air into the engine allows for more fuel to be dumped in while maintaining the proper fuel to air ratio. More air + more fuel = more power. This is an old school view to increase power and still applies even to today's complicated computer controlled engine bays.
So...we want to increase air supply which also allows us to pump in more fuel at the same time.
So...let's look at non-mechanical ways to increase air (and fuel!) into the combustion chambers.
1. One way to increase air in is to make it cooler since cooler air is more dense than warmer air. Hence CAI's taking air from as far away from the engine as possibly. These do work, but are limited...since we drive during the summer the air is still relatively warm and therefore their usefulness often doesn't meet the claims. But, they do help.
2. Another way to increase air into the combustion chamber is to exhaust all the air that was put in during the prior intake stroke and ignited during the combustion stroke. The way we exhaust more air is to reduce the restriction on the exhaust. Hence....headers and more free flowing mufflers. These again will help but the gains are limited if everything else is stock. Combined with a CAI, they are useful. IMHO putting on a set of headers, less restrictive mufflers, a CAI, and a throttle body (as described below) followed by a tune is the most economical way to get HP and easy to install. Long tube headers are best. With this said, the stock GT exhaust is actually pretty good at exhausting gas and if you look, they actually are a set of headers...but remember that production cars consider cost and mileage...so they aren't optimized for power.
3. A very useful way to increase air into the combustion chamber is to increase the size of the opening at the front of the air intake. This is throttle body area. Oh boy is this an area. You have to research what you are looking to accomplish when looking at throttle bodies. Do you want low end torque or high revving power or mid range power. This is going to require research on your part. If you have a daily driver, mid range power and fuel economy may be primary concerns...etc etc.
4. Intake manifolds will allow air to travel more efficiently to each cylinder. As air is bent through different tubes or channels it creates resistance. That resistance reduces the amount of air able to be sucked into the combustion chamber on the intake stroke. Putting on a new manifold with a different design changes the flow of the air and improves efficiency. These manifolds are as diverse as throttle bodies and you have the same considerations (low range, mid range, or high revving HP goals).
5. Injectors....this is how you get more fuel as you add more air. Research will get you the right size injectors depending on how much extra air you are pumping in.
6. A crossover pipe helps equalize exhaust pressure and is quite useful for that purpose. However, I'm not certain the claims are as real as stated in benefits for a car that is not boosted.
7. Larger radiator and perhaps an extra fan or two. Don't forget, as you increase HP you increase heat within the engine block which both robs power and can damage the engine. We want heat in the compression chamber but then we want to remove it from the engine block and heads as quickly as possible.
8. And lastly the "tune". Us old timers used to have it easy. We'd change out plugs to get hotter or colder and literally rotate a rotor with our hands to advance or re tard (had to break the spelling as the message board doesn't like the word r e t a r d....that's odd) the timing (mostly advancing for more power). Depending on how hot we wanted the compression burn (with the consideration we didn't want to 1) melt a spark plug electrode 2) burn a hole in the piston 3) crack a head) in order to burn as much gas as possible, we would turn the rotor clockwise/counter clockwise. Nowadays that is all done with the computer. Even with a stock motor, you can play with these settings and increase power if you increase the octane of the fuel you are burning. But...this is best left to someone who knows what they are doing. As you change anything on the car you likely will have to change the computer settings. WISE PEOPLE change one thing at a time and retune the car with each change. It may cost more money, and certainly takes more time, but saves in headaches (changing multiple variables at once is never a good idea because it makes chasing problems infinitely harder).
So, if you want 650HP+ you can do it with a naturally aspirated motor. And it will costs thousands of $$$ LESS than slapping on a S/C, but it will take a lot of research. If you install a S/C properly you end up with the best of everything....lots of extra air in those cylinders at low, mid and high range. If you install a turbo its a bit trickier but still a lot of power (maybe even more since you don't have the scavenger loss of power from running the super charger impellers using the belt drive off the engine crank shaft).
If you do decide to go the naturally aspirated route and start working with the items above, you need to do your research very carefully as everything needs to work in harmony with the RPM/HP goals you are trying to achieve. But, if you do it right you can get a ton of power without the maintenance of mechanically forced induction and do it for a lot less $$$$.
Phew....time to go to bed.