Maybe spam's other points are the real issue here. Still, I like his comment about validating the thermostat you have in the car. I had intended to address this point, he beat me to the punch. For all I know there are multiple threads on TDIclub and elsewhere on the web addressing this. I'm just thinking about it, so I'll lay out my perspective.
You might feel I'm laying out a wild goose chase. Or, you might feel I'm laying out how you can eliminate possibilities so you can narrow the focus of what's really going wrong.
Validating thermostats:
Hopefully you got the old thermostat. If you didn't, it's just something to think about in the future. If you did, is it fully closed at room temp?
Did you see the new one before it went in, was it fully closed at room temp? Although you'd like to think the mechanic would have cared enough to notice this . . .
Further, it's an easy thing to put a thermostat in a pan of water on a stove/range with a cooking thermometer, turn the heat up and down, note the temp when it cracks open, when it's fully open, when it starts closing, when it's fully closed. Basically, get an idea of how it's really working.
And. If you do your own work, and you think it's time to replace a thermostat, consider this. Before you go to the time and trouble of putting the new thermostat in, by the method above you can verify it's operation is different than the last one, and hopefully closer to how you really want it to work.
At the least, you can get a new thermostat now, check it, and then swap them, and check the one that you just removed. If you still have warranty, your choice, of course, whether you want to go to all of the trouble.
Yeah, it's not supposed to be necessary to do this. Life just works that way sometimes . . .
If anyone feels I've made a mistake here, please let me know, I'll be happy to come back and correct or delete.