More information there, DEFCON talk of someone reverse engineering the ECU and then discovering the actual data that corresponded to the EURO5 emissions test : https://www.youtube.com/watch?v=xZSU1FPDiao
They have their own video streaming service which doesn't do the tracking Youtube does:
https://media.ccc.de/v/32c3-7331-the_exhaust_emissions_scand...
They could easily reverse it back, the power will be the same, pollution levels will be okay, only AdBlue/Urea consumption will increase.