There was a thread on here a while back. It was in regards to lower arms. Basically it asked is there a specific 'turbo' lower arm, which was confirmed as there is. No one knew the reason, some suggested that it was double skin, however the turbo arms aren't double skinned.
Talking to a friend today who has a normal mk4 we have now found the difference, the turbo lower arm is a slightly different length on the rear mounting point, this gives the turbo models more castor. This creates nicer turn-in, more self centered steering and dynamic camber. If anyone has both the arms can you just check to confirm this, as it may be that he's had aftermarket arms put on that aren't a great pattern.
Or anyone with autodata check the turbo alignment settings compared to the standard ones.