I've a GLI 2010 model and i had been suffering from the same “AC's-less-cooling” issue. Like on high RPM it was working good but not when the car slowed down.
So, i went to a professional AC technician, he checked every single pipes and everything related to the cooling system of my car. Then he advised me to change the cooling coil of the car. He installed a 'Japanese made' cooling coil of vitz 2006-7 model (which works better under the low RPMs too)
Believe me it's more than a year since I've been using it and the AC is working superbly, like “Chilled”