Stephan Reil, boss of Audi's Quattro division recently told in a interview with CarMagazine that they will soon introduce a "Plus" version of the RS6 Avant with over 600hp. That will be an increase of almost 50hp! Reil also added that he wants to save Audi's high-performance naturally aspirated engines while also making future RS cars lighter.
In 2004 Audi created a RS6 Avant Plus with several improved parts, like the brakes and suspension, and more power, but it was a limited edition. Since then Audi introduced the TT RS Plus and the R8 V10 Plus which are full production models, so the new RS6 Avant Plus won't be a limited edition.
The future Plus models will have more modifications besides just more power. One of the main objectives is to reduce weight with the use of carbon fiber and in some cases aluminum.