you need to start *** flirt with these mods simply because the engine mods wont provide the required results if the exhaust is restricted
This is an edited reply (after I did some research...). My earlier comments were incorrect.
While freeing up the exhaust on a turbo would
improve it's performance, there are some other things you can do that will have a greater impact at lower cost. Specifically, you can spend $1000-1500 on an exhaust to pick up 10-15 HP out put in a new a chip and gain 30-40 HP for <$800? If you're talking about a 4.2, you want to be careful about opening up the exhaust. Naturally aspirated engines need the backpressure of the OEM designed exhaust.
To the original poster... The link I provided offers the opinions of people on this forum (to the extent many people on this forum are also on AudiWorld). Trust me: you will NOT find a better collection of commentary on modding an A6 then the link I provided for you. If you want to do the reading, there it is. If you want people to hand you the info, you're going to have a harder time. Your question is very general and those sorts of questions do not tend to get as much response as more specific ones that come from having done some background reading first.
Asking "what sort of mods have you done?" is like asking "what sort of women have you dated?" You're going to get LOTS of different answers that will not have much meaning without some background context.