HOXXOH

Here's how I did it on my '08 A6. Notice I removed the manifold studs and replaced with bolts. When I did swaps for a couple friends, I learned to keep one stud (the easiest to access) to assist in installation. I also made the mistake (as you can see) of welding the 2.5" pipes directly to the 3" pipes. For my friends, I cut the 3" pipes off about 10-12" from the "H" and put in a 3" to 2.5" reducer and then cut the OEM mid pipe so the hanger matched up the length correctly. I used clamps on the reducer, so no welding was required. We did all the prep work before installation. It took about 4 hours on jackstands, but for a couple guys in their 70's who are in no hurry for anything, that time could be less than 2 hours for one young guy using a lift.

One additional thing you might consider, is to weld in a bung on the right bank for a future air/fuel gauge. You can put a plug in it for now, but it's a lot easier to do it now than later. Note the location in my photo.
BTW, I managed a 10.82 @ 125.82 with this exhaust setup.with just bolt-ons.

Guyzer7

Thanks for passing along the installation tips!