Sorry, dont know about the microchip thing, but regarding a (mailorder or otherwise) tune :
Its not only about "adding " hp, but also about stopping the computer control in the stock tune from limiting (or taking away) whats already there.
The C5 is a wonderful platform, but it is also a mass market car built for regular people, not "car people" or "experienced" racers in its stock form.
We have a very safe system for a car with the inherent amount of power and potential for "delta V" (thats change in velocity

both acceleration and braking) and it (often) keeps people out of harms way.
If you stomp on the throttle in a regular (as opposed to drive by wire, like the C5) car the throttle opens all the way, its linear. In the C5, it opens the amount it feels you need for a safe and unchallenging (although spirited) take off. I dont remember the numbers, but it could be like 10% (in other words when you nail it it opens 10% at first to keep you in control, until your moving faster/farther, then opens more.
We also have stuff like tourque management (which compares what you are telling the car to do with data in a table and if it appears that you "could possibly" harm a component in the drive train, it will retard the timing so the engine puts out less power) and traction control/active handeling (pretty sure they both work taking wheel speed and inertial sensor data and comparing that to a table again to make a determination if what you are telling the car to do "could possibly" put you in a situation where you would not be in control, or that you are in fact out of line (based on an average drivers skill set), and applies individual braking (one or more wheels) to correct the condition. I dont thing the stuff changed in a "tune" touches the active handeling stuff though. But it often effects the torque management parameters.
So . . . . a lotta stuff goes on when we press down on the skinny pedal, and the car and its computers are always listening. They intervene often.
A tune can change a lot of those parameters. The very conservative parameters as the car is programmed from the factory do leave a lot of room for more power and acceleration while still maintaining safe operation. The reprogramming could allow the engine to make more horsepower in certain situations, open the throttle more, retard the spark less, shift the transmission (in autos obviously) faster, a bunch of things.
This is all based on the experience of whoever you allow to tune your car. You allow a shop thats done literaly hundreds of tunes on C5's to reprogram your pcm, and you will likely wind up with a very noticable difference in the way the car performs.
It goes the other way also, as most things do

, meaning if you allow "your brother-in-laws cousins stepson, fresh out of vocational school" to mess with the program, you may wind up with at best a broken car and at worst a broken body.
So the mail order tunes, including the ones where you provide a detailed account of what you are looking for and any mods to the car can make a big difference in how the car performs.
The next step (not that anyone asked

) would be to have a "wideband" street tune (where the car is connected to a host of monitoing equipment that allows the tuner to see (in real time as the car is being driven) what the car is doing) or a dyno tune (same principal, just that the car is on the dyno connected to the monitoring equipment rather than being driven on the road). Those two situations should lead to a much more refined set of parameters for the tuner to store in your vehicles system, and greater performance gains.
Can you tell I just had this done ?

otfl
I had been sitting on the fence for a long time. I felt the car was plenty fast and performed better than anything I had driven before. I dont race, I didnt seriously consider (well I considered but not seriously enough to spend the money) modding for speed yet (heads, cam, stall, drivetrain...whatever) and also didnt expect my stock 2001 A4 (just a blackwing and catbacks) would benefit from a tune.
I was wrong.
I had a street tune done and I can tell you it does drive like a different car. I can break the wheels from a standing start at will where I never was able to do that before (not looking to burn up tires, but it does illustrate the increase in power that is useful in other situations).
Just a brief description of what a "tune" does.