First we'll talk about what you can do from inside the hemi car to modify it. The tuners and programmers are installed through the computer port that is just below the steering wheel. They add about 20 horse power and usually about 1 to 2 miles per gallon to the fuel economy, just because of the improved engine efficiency. The hand held devices also give the ability to read and clear the check engine lights, saving a trip to the dealership and some time. They also make way for future upgrades with the ability to get custom programs from the handhelds manufacturers.
The second place to modify is under the hood by replacing the factory air box with a more efficient air intake system. The aftermarket systems have much larger filters and are able to suck a lot more air into the engine to create power, and make it run more efficiently. And the third place is under the car. Upgrading the exhaust will do a few things. The most noticeable will be the different sound. You'll end up with a more aggressive tone. The other thing is does is free up the exhaust flow. This will help the engine to run cooler. It also help the engine not have to work to push exhaust gasses away, because they are more free to flow through the pipes. This will also improve engine efficiency.
Why's doesn't the manufacturer do all of these upgrades automatically. Think about that question for a second. Think of the behind the seat dvd players that the aftermarket came up with that would strap on with velcro straps. Now you can see built in dvd players from the factory. Now think of the wheels and suspensions that the aftermarket came up with, and now take a look at some of your local dealerships, and you'll see that they are catching on to these things. The aftermarket is what makes the auto industry evolve. The car manufacturers then take what looks like it will be most profitable and then incorporate those things into their cars. The aftermarket industry will always take what they make and make it better.
Installing Air Intakes and Chips on the hemi cars can easily be done all by your self with simple tools. The tuners upload in under 10 minutes and the intake usually take about 25 minutes which isn't bad for getting years of extra power and efficiency. The hemi car exhausts can be installed at home as well but will require a little more skill than the Intake and Chip.