We had a few '00 A6s in our family; I detailed them and had a 4.2 of my own.
The clear is very hard. To correct it with a PC (in a reasonable amount of time) you'll have to use an aggressive product on a 4" pad. I spent literally *hours* with 3M PI-II FCRC/PC/6.5" wool on the trunklid of an A6. After basically a full day, I'd barely touched the marring. That taught me that the PC/6.5" approach simply isn't suitable for that clear.
Once you get the worst of it out with the aggressive product you can try switching to something milder on a 6.5" pad but be prepared to switch back to the 4" set up.
Product suggestions: 3M PI-III RC 05933 (discontinued but still available) followed by PI-III MG 05937 (ditto); 1Z Ultra/Extra followed by their PP (following with their MP was worth it for me); Hi-Temp stuff (no first hand experience on that paint).
FWIW, I've used the following approach, *by hand* to remove fingernail scratches from behind door handles on this clear: 1Z Ultra, then 3M PI-III RC, then PI-III MG, then 1Z MP. Took a *very* long time but it worked.
Don't use something that the PC won't break down; if you get product hazing from something rotary-only you might not get it out with the PC.
DO NOT worry about taking off too much clear
Plan on spending a lot of time. Work one panel/area until you get what you're after.
I got by without a rotary for a long, long time but finally bought one to correct not-awful marring on Audis
