Very odd malfunction: misfire after tire change and front alignment

Today I took my B5.5 for a tire change. Two weeks ago the car was tuned-up and had an oil change. Everything fine, smooth and excellent fuel mileage (30.1 mpg). Suddenly the car began with a misfire when the installer handed me the car. I asked him, WTF?? The obvious answer you might guess "oh no sir, we just changed tires and did a wheel balance and alignment". The car crawled home as if just 3 cylinders were working (this being an 1.8 engine). Fuel mileage came drastically down and feels as if the turbo went on a sudden holiday! It is already dark out there and I have no VAG or scanner at hand in my actual location. I prefer not to take the car out in such circumstances. Anyone has a clue of what might be going on?

I might say that in the recent oil change, due to some confusion or lack of knowledge, the guy at the oil change garage overfilled slightly the oil deposit. When I went there to pick it up I checked the indicator and it was way past the full mark. Up about 200 - 250 mililiters. I asked the guy to drain some of the oil to bring it down to the specified quantity. I asked if he drove the car or for how long did he kept the engine running. Said about a minute to check for leaks and then some parking manouvers. That makes about 2 minutes. I begin wondering if my CAT has been killed

Would appreciate some helpful tip or similar experience.

Regards!