There are not many mods for the 4.2. A chip won't yield you much HP because it is a natural asperated engine (hopefully I spelled that right), meaning its not turbo'd or supercharged. You can add an air system and exhaust. It will help it breathe and sound better but still the HP will be extremely minimal! The horse to modify is the 2.7T! You can check Milltek for the exhaust. GIAC, APR or AWE Tuning for chips and LLTEK. Good Luck!
It is actually my boss's car. I have an S2000 and there is a ton of modifications avalible for my car. He asked me about modifying his car and I told him I would look into. I am a little out of my element in the audi scene. Initially he wanted to get a chip and I told him on N/A they won't get you much because they can't change the boost. I looked at almost all the companies you listed and none offer anything performance wise for the 4.2l. I will tell him his first mod should be selling the 4.2 and get an RS6.
It is possible to reach more horsepower and torgue with chip tuning regardless of that your car doesn't have a turbocharger. It is possible to increase a power about 14 kW with 24 Nm increased torque. At the same time, a top speed increases a little bit and a fuel consumption decreases as well.
And yes, I know that that kind of power and torque increase is almost nothing, but it is possible.
Please visit a following link. It is in the Netherlads language, but surely you will get the point.
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