In the case of dieselgate, they weren't going to stand still. To shed some light (or more uncertainty) on how Volkswagen will fix cars with affected engines with the software of discord, Consumer Reports road-tested two Volkswagens with, according to them, the software running in "EPA homologation" mode.
They put a 2015 VW Jetta DTI and a 2011 VW Jetta Sportwagen TDI head-to-head. They measured acceleration and fuel consumption on the road (following their usual test track).
The result between "normal" mode and "EPA homologation" mode doesn't differ much in the case of the 2015 JettaTDI. It loses just 0.1 s in 0-60 mph (96 km/h) in "trap" mode (an irrelevant difference) and its consumption increases by 0.3 l/100 km (4.4 l/100 km in normal mode and 4.7 l/100 km in trap mode).
The case of the 2011 Jetta is more revealing, especially in terms of fuel consumption. In acceleration, it loses 0.6 s; it's not much, but you can already notice a slight difference. In consumption, however, the difference is already more noticeable. From a highway average of 4.7 l/100 km in normal mode, the average rose to 5.1 l/100 km with the "cheat mode" activated.
If Volkswagen's solution to bring its cars into line with current legislation is to activate the "homologation" mode by default, very few people will go to their dealership when it's time for a recall.