Your Audi already has anti-sway bars (front and rear). On the standard suspension set up they are adequate, but some would argue that they are of insufficient diameter. A larger diameter bar will give you more resistance to body lean. Upgrading a rear bar will make an even greater improvement than just the front. By keeping the vehicle more level in a turn you will have significantly more control, giving you better handling. A good combination of front and rear bars can also keep your car more stable on the open road. This plus your new springs will make your A6 feel like it is on rails.
Unlike springs, which can be noticed, sway bar upgrades will be virtually undetectable except for the imrovements noted above. Oh and they are inexpensive, too (as I recall, new front and rear anti-sway bars for my 1997 A8 were a couple hundred bucks -- I bought the S8 suspension upgrade from Hoppen, BTW, as Audi was not marketing upgrades on their web site at that time). Contact Joe Hoppen http://www.hoppenmotorsport.com/ for more details and a technical explanation of what these things do. You will need to call them as the anti-sway bars are not listed on their web site.
When you put your new springs and roll bars on (let the dealer do it) -- the dealer will perform a full all wheel alignment. IMO you will need a second all wheel alignement after 1,000 miles. Your tires will thank you for the second alignment. Reason? The springs will "settle" or could do so and generally the second alignment is included in the installation charge. It was at Northland Audi here in Cincinnati, anyway.
Hope this helps.