V8 Vantage Exhaust System
Exhaust systems are fairly standard items for every car. You've got an exhaust manifold, catalytic converters, pipes, and a muffler. On turbocharged cars you'll have a slightly different setup to accommodate the turbo, but the rest is the same. The V8 Vantage exhaust is a straight-forward system, so let's go over each component and the pros and cons of modification.
DISCLAIMER: As always, follow all safety protocols. Don't undertake this task if you aren't comfortable with it and fully understand it. You are ultimately responsible for anything you do. Neither Redpants, LLC or myself is responsible or liable for anything that may occur. The comments below do not take into consideration certain restrictions, such as Federal emissions regulations, local noise ordinances, etc. It is your responsibility to comply with any applicable laws.
To begin, here's a video where I discuss the exhaust manifolds and catalytic converters of the V8 Vantage. A bit of a correction to the video, though: The extra set of catalytic converters wasn't integrated into the headers when the 4.7L V8 was first introduced. It was done sometime later, around 2012 or so.
Exhaust Manifolds (AKA Headers)
The first component in the system is the exhaust manifold. It takes exhaust gases from each cylinder and combines them so they can flow together in one pipe. Here's a picture of one of the OEM exhaust manifolds from my 4.3L V8 Vantage (top) and one of the exhaust manifolds I replaced it with, a 4-to-1 equal-length exhaust manifold from VelocityAP (bottom).
Aston Martin added catalytic converters to the headers of the 4.7L cars to help further reduce emissions, especially cold-start emissions.
Are they worth changing?
It's hard to say definitively whether or not someone should replace the headers on a 4.3L engine. You'll get a little more power from a 4.3L engine, and a little better performance all around by switching to an equal-length design. About 9 lbs is also dropped from the car's total weight, which is an added bonus. But the high cost and difficult installation is a serious detractor unless you're intent on keeping the car. For a lot of people, trading the car in for a 4.7L car gives more benefits at a relatively low cost. You can also do the same modifications on a 4.7L car, which has a higher performance potential due to the larger engine. Since aftermarket headers eliminate the catalytic converters in the later 4.7L's OEM manifolds, you'll gain even more power by switching to them on those cars and it's a much more worthwhile modification to the newer, larger V8 engines.
Catalytic Converters (AKA Cats)
Catalytic converters ("cats" for short) are the part of the exhaust that affect emissions. These have a catalyst material (thus the name) that reduce amount of polutants coming from your engine before the exhaust gases are released to the atmosphere. Here's what an OEM cat looks like, along with its o2 sensors:
The OEM cats are 600-cell units. The cell count is a measurement of how much catalyst is in the unit. However, the o2 sensors are only metering the efficiency of the first third-or-so of the catalyst material. In the picture above, you can see that the first o2 sensor (the one on the right with the green connector) meters exhaust gas as it first enters the cat. The downstream o2 sensor (the one to the left with the blue connector) is placed near the middle of the cat, which means it can only get readings for part of the unit. Gases still pass through more after the second o2 sensor takes its reading, and those gases get further cleaned, but the car's computer doesn't know that. Because of this, we can swap out the 600-cell cats for better-flowing 200-cell units without triggering a check engine light (CEL).
Are they worth changing?
Yes! Replacing the OEM cats with high-flow units will get you around 15 hp, give your car a more aggressive sound, and drops several pounds of weight from your car. One the 4.3L engine, I'd suggest 200-cell cats, which is typical for this application. They might be called high-flow or race cats, but they're all pretty much the same - a pipe with a smaller catalyst unit built into it. On the later 4.7L, you can eliminate them entirely because there are still cats in the headers (so long as you still have the OEM exhaust manifolds, that is).
Mid and rear pipes
These pipes are just that - pipes. They carry the exhaust gas from the catalytic converters to the muffler. They do their job and they do it well, so there's nothing to know about them other than where they are, as you may need to loosen or remove them while working on other things around them.
The mid pipe (or "center pipe") is the one that starts at the rear end of the catalytic converter. It then couples with the rear pipe (or "end pipe"). The rear pipe goes from the mid pipe into the muffler.
Are they worth changing?
No. There's no reason to change them unless you're doing some seriously extensive modifications... like side-exit exhausts that eliminate the need for them entirely. So no, just know what the mid and rear pipes are and don't worry about changing them.
Muffler (AKA Rear Exhaust)
Most people call the muffler the rear exhaust. I think it's because "muffler" has a cheap connotation to it. It's also the part that people are most often referring to when they ask what exhaust you have. The OEM mufflers have valves built into them. Under a certain RPM and throttle load, the valves are closed. This routes the exhaust through baffling that makes the exhaust quieter - ideal for during normal driving. When you hit a certain RPM, the valves open up. The open valves allow the exhaust gases to go through a more direct route, bypassing the baffling that would otherwise reduce noise. This gives you a louder exhaust note when you're pushing the car.
If you want to keep the valves open at all times with the OEM muffler, you can pull a fuse in the trunk-mounted fuse box. The infamous fuse 22 can be pulled out to keep the valves open, and replacing it puts everything back to its original design. In later cars, the fuse was moved to slot 15 - so if fuse 22 doesn't work, try fuse 15.
Is it worth changing?
Yes! You'll gain about 3-5 hp, which isn't much given the cost, but you're also getting a huge reduction in weight and a more aggressive exhaust note. Typically, your exhaust is going to be louder with an aftermarket muffler, so keep that in mind if noise level is a concern. Many aftermarket mufflers eliminate the exhaust valves found in the OEM muffler. You'll get the louder exhaust at all times rather than the OEM setup's quiet at low RPM/loud at high RPM noise levels.