My son owns a 2013 model Mk6 that has been very slow to warm up. After reading lots of posts on the net, there are 2 thermostats for cars with DSG. We originally purchased the main thermostat & were very discouraged by the procedure & time to replace it. Some had recommended to replace the trans cooler thermostat first as this was the far quicker replacement. Some said it was a 10 min job that was rubbish, more like 1 hour for us. [We drained the coolant]
It was found near the trans cooler at the mounting face between the engine & gearbox. Take note of which way it is installed before removal.[arrow on thermostat housing]. Don't forget to reinstall rubber sleeve before connecting hoses. [we did]
Car now warms up as it should.

Before I installed the new stat. I ran some cold water through both old & new. Water still passes slowly through the new one meaning it doesn't shut of completely. The old one flowed through quite freely. I pulled the old one apart & ran some hot water over the brass working part that made the pin extend probably to 12mm. On cooling it didn't retract so a sticky pin was the problem.
I hope this helps someone.