The big thing that often gets neglected is actually getting power to the road. You can have all the power in the world, but if you can't make it stick it's worth exactly #### all. Good suspension set up, including shocks and bushes, to keep the rear end under control, a good LSD and good tyres will help massively with pace off the line. Then you want to focus on your torque.
Yep definatly diff gears, the 3.45units out of the manual cars seem to be a perfect fit for the A6. I know of 2 people who have the 3.7diff's and both have different opinions on them, one hates the very very short 1-2-3 but the other guy loves it. Doing a full exhuast inc some big 4-1 headers & high flows cats, combined with an otr & tune will go well with the shortened gears.