The usual rule of thumb with this Audi is that it is slowest before making a pitstop out of the 4 cars, but the fastest after. The correct setup can make it go just as fast before and after the pitstop.
As far as I was aware, (haven't paid too much attention to rule books), in single class competition, like F1 or Touring Cars, even GT's, a waved blue flag means move out of the way, you are about to be lapped.
However, in multiclass racing, IE WEC, Tudor, Le Mans etc, you don't have to move for cars in a different class. So if you are in GT, you don't move to let the Prototype through, but, if another GT car in the same class as you comes to lap you, you still have to move over and let them through.
On another note @
CodDemon101 -- are there any spots open in the Audi? I have a friend who is interested in joining. He isn't on GTP yet, but he's a good lad, fast and clean. I think he might try and contact you later.