This is the official description posted to the page before it was taken down, “The RS6 Plus is the latest of Audi’s stunning RS ‘Plus’ models, offering even greater performance and agility than the standard RS6—on or off the track. The RS6 Avant Plus uses a tuned version of the 4.0-litre V-8 TFSI RS6 engine which produces 600 hp—40 more than the RS6.”
The other interesting part beyond the power boost is that Audi apparently is shaving weight from the RS6. How much weight? We do not know but it is nice to see they are not just focusing on the power alone.
What we learn is that much like the competition Audi can offer a performance package option to increase the horsepower of the 4.0 TFSI V8 in the RS6 and RS7. An RS6 Plus model likely means an RS7 Plus model as well which is good for US fans as Audi only imports the RS7.