Originally Posted by TryHard

I can personally confirm that in practise they were all (Bruno, Alex and JJ) switching between std and ARS cars each run. Mainly as I was pitting alongside the Xray team all week.
Come Control practise, qualifying and finals (with a single exception) it was ARS only.
Did hear some rumours that there was a quick switch to the standard rear end on at least one car for A3, but can't confirm that for sure mainly as I was watching finals whilst the cars were being prepped.

The general feeling was that the ARS was quicker as it gave better low speed steering for the tighter sections of the track. And the grip level was high enough to help the consistency. So most Xrays in the pits tended that way.