The WALK only adds half a degree to an otherwise non-adjustable castor setting. So just stick with the 'early' OEM GTI settings. VW did later release some supplementary GTI settings, which basically lessend the rear camber for peeps who mainly tramp.
And are you having it done at Wheels In Motion HQ, or one of their 'agents'? If using an agent - be very careful - WIM use an aweful lot of 'Stapletons' tyre depots - but they are a bunch of incompetent w@nkers!
If you have coilovers fitted, I would rather use Centre Gravity, rather than a WIM agent.