Using a 2015 VW Jetta TDI and a 2011 Jetta Sportwagen TDI for their test, they reckoned that they managed to trick the cars into 'believing' they were on cheat mode while driving on the street.
With the 'cheat mode' turned on, both cars saw a drop in fuel efficiency and performance. The older Jetta Sportwagen TDI was 0.6 seconds slower from 0 to 96km/h, while fuel economy dropped from 21.2km/L to 19.6km/L on the highway.
The newer Jetta sedan loses only 0.1 seconds in the acceleration test, but drops from 22.7km/L to 21.2km/L in the economy test.
In conclusion, if the way to engage the 'cheat mode' is similar to what is shown in the video, then the real life difference isn't going to be big at all.