The car company engineers do a really amazing job of making modern cars high performance and also low pollution. Modern catalytic converters and variable valve timing and computer controlled feedback loops with air sensors are real technological marvels. And yet some redneck with a blow torch thinks he can improve it by cutting some holes or putting on fatter pipes.
First of all, "freer flowing" or "reduced backpressure" is rarely a good thing to do to your car. Putting on fatter pipes rarely helps performance much, and if it does help it probably helps only at high RPM at the cost of performance at low RPM. The reasons are rather complicated, but basically the combustion chamber is designed to have just the right amount of exhaust exit flow. What that should be exactly depends on the car and the RPM, but typically you want the flow of exhaust to create a vacuum to help evict the cylinder after combustion; some engines also want the exhaust to stop moving and create a sort of wall as the exhaust valve closes and new air and fuel comes in. Changing the diameter of pipes can screw up these pressures, and the result is often torque loss at low RPM. At high RPM it's a simpler situation because the cylinder is exploding so many times and making so much gas you just want to get it out as quickly as possible.
The latest trend in car mods is to do a full or partial bypass, just cutting a hole before the cats that dumps exhaust straight out the back of the car, sometimes on a switch, sometimes not. Even if it did help, it would still be dickish to ignore your fucking up the air quality just because you need 5 more horse power. Some of the modders are just confused because cats actually did used to hurt performance back in 1950 or whenever they first came out, but modern engines and cats work marvelously (very large high reving engines like race cars can still see a small benefit from not using them).
But worst of all its just super douchey to make your car louder. I personally love to hear my engine, and actually I wouldn't mind it being a bit louder, but I want it loud just for me in the cabin, not for the whole rest of the world (and I'd like it on a switch too, so I can have quiet mode and loud mode). People who amp their exhaust are obviously doing it intentionally to be obnoxious. Hey pedestrians walking on the road, listen to this! Yeah fuck you for trying to have a conversation, look over here at my great fucking car! My penis might be tiny but my exhaust is huge, oh yeah!
In other douchey car news, my car has xenon head lights and I hate it. I'm kind of tempted to "smoke" the headlights just to reduce their brightness a bit. I can see people in front of me get annoyed when I pull up behind them. Of course I'm quite low so my sin is only one tenth the sin of a high-carriage car with xenons, but I still hate to be that guy. (In other "that guy" news, I used the carpool lane through Bellevue for the first time. I have no regrets).
ADDENDUM : another awesome example is what people do for air intakes on the 911. There are a few varieties of stupidity here.
One popular mod is like Fabspeed smooth red tube . The OEM air intake tube has a few very clever features; for one it is baffled to reduce noise and drag (contrary to Fab's claims, rough surfaces actually make less air resistance than smooth in many cases; see eg. the golf ball; the rough surface will create turbulence while the smooth surface will build up a big sticky boundary layer; of course the reality and details are incredibly complex and it's hard to say what's better).
But the main feature is the little tube that comes off the OEM intake. That little tube is a Helmholtz Resonator, which vibrates at a certain frequency as air passes over it (just like blowing over the mouth of a bottle). It's tuned so that that resonance cancels out the vibration resonance of the air coming in at low RPM. At high RPM they go out of tune and no longer cancel. The result is the car is nice and quiet at low RPM and then howls at high RPM. (see here for example)
The main thing that the air intake mods do is just break those clever features. Of course they could get the same effect by just putting a piece of duct tape over the resonator tube. And hell while you're at it stick a playing card in your air intake so you get some extra noise.
To some extent the whole "cold air intake" thing is another left-over from the 50's modder days when manufacturers actually got things wrong. Back then you had carburated cars taking big gulps of hot air from inside the engine bay. If you just stuck on a tube that ran the air intake out to the outside of the car, it would let it breathe cold air, which increases engine efficiency (cold air is denser in oxygen mainly). Modern cars all route cold air to the air intake very efficiently, either using their own tubes or simply through controlling the air flow patterns when the car moves. Modern cars are not designed to breathe correctly when they are sitting still (eg. on a dyno) which is part of how these nonsense tuners can claim gains on the dyno (the main way is just faking the results, eg. running the "before" test on a cold engine and the "after" when it's all warmed up).
Anyway, it's a cheap way to get a little sound, which is what buyers really want. The funny thing is they can't admit that, everyone has to pretend it's about performance. Both buyers and sellers happilly go along with the fraud, sellers post fake dynos and buyers claim they "feel a difference". (kind of like how ballet is really about seeing some hot half-naked people, but everyone has to pretend it's about the story).
Another popular mod is a BMC or K&N cotton oiled air filter. These filters do seem to in fact let in a tiny bit more air (maybe 1% more) which might increase power, but they also let in more particles, which might hurt engine life. It's almost a tautology that more air flow = less filtration, unless you actually increase the surface area of the filter. (see here for example). There's also some indication that those filters let in more air when clean, but less air when dirty (compared to a standard OEM paper filter). Many ricers want to use BMC or K&N filters because "racers use them". It's a completely different situation for a few reasons - 1. race engines have a lifetime of less than 1000 hours, so longevity isn't really a big concern, 2. they actually need as much air as they can get, most road engines are not air limitted (or rather, they are, but the problem is not availability of air at the intake, it's driving it into the cylinders, and the solution to that is forced induction, not filters), and perhaps most importantly 3. the racers are sponsored by those filter companies.