The rear ARB (anti-roll bar) has a part number that begins 5q0511305b, with another letter after the b, from a through to at least o. I believe that this letter determines the stiffness of the bar and its application.
 

Does anyone have access to a list of the variants and their application, and even better would be their specification (overall thickness, wall thickness etc)? I would like to purchase a stiffer one than the standard but still a stock VW bar rather than an aftermarket bar.

Answering my own question, here is a list of some of the ARBs fitted to AWD MQB cars:

 

5Q0 511 305 BC - 19.6x2.6
5Q0 511 305 BD - 20.7x2.8
5Q0 511 305 BE - 21.7x3.0
5Q0 511 305 BF - 21.7x3.6

 

I believe that the standard Golf R bar is the third one in the list, also presumably fitted to the VRS 4x4. The first one is what is fitted to the Scout or equivalent and I think that the second is fitted to SE 4x4 models. I would be very grateful if anyone with more knowledge could confirm or correct this information.