The center diff on the RS6 isn't an open diff, it is a crown gear, or Torsen diff. The slip/Torque ratios can vary depending on the gear in the diff. I think the standard RS6 is a 2:1, and what you are probably feeling is you probably had a 4:1 installed, I did too Driven a couple older S4's and UrS6's with the 4:1 and it surprisingly has to be one of the best handling upgrades for sport driving.