Tox, you are right about exhausts being more then just loud but there is something you are missing. On a stock out of the factory engine any gains you may see from an exhaust will be minimal at best. Believe it or not some backpressure can be a good thing if you want to make torque and horsepower you can use on the street. Installing a dual exhaust is a sure fire way to kill the bottom end torgue on a truck. you MAY gain some horsepower but at a high rpm that you will never use in the real world or you can even loose some. If you want high rpm horsepower a free flowing exhaust is the way to go but the motor needs to be built to take advantage of that. if you want torque you need some backpressure and the way the trucks are built today the factory stuff is pretty good. There is a reason the Nascar guys run mufflers on thier cars, they figured out that they make better power with them then without.
People need to keep in mind that a motor can only flow so much volume thru it( think as in an air compressor) and no matter what you do on either end it still can only pump so much. To be able to truly benefit from that extra flow the motors volume needs to me increased as well. Things like head work and cams help that.
When i was going to college i worked at a friends speed shop building motors and cars for people. Having built lots of motors and run them on the dyno i can speak from experience. I can also give you a great example of backpressure as it relates to torque. I had a 93 gmc sonoma with a 200hp 4.3. I decided because i couldnt afford anything else i would hop it up and make it my drag car. The motor got a lot done to it head work, cams, different pistons, rods, crank, injectors, headers etc. I wanted my power to come at a higher rpm so i installed a free flow muffler and yea it helped but the motor was highly modified. I was running in the mid to low 13's i nthe 1/4 with a v6 truck all motor( no nitrous). I decided to try removing the cat to see what almost no backpressure got me and when i put it on the dyno to see where i stood i saw i lost so much torque down low that the 2 hp gain i say at 5500 rmp wasnt worth it and back on it went. That bottom end torque i lost would have actually slowed down my times.