Just had a letter stating that my 500x cross plus 1.6 diesel is part of Engine Control Module software update.It is to re calibrate the ECU to lower the Emissions for Euro 6 in 2017
Reading between the lines it similar to the VW fiasco.

Has anyone had it done yet?

If so has it affected mpg and performance?

It's definitely possible. The cars should already meet euro 6 standards from the factory, but maybe they are preparing a tweak before Sept 2017

http://uk.reuters.com/article/uk-fiat-chrysler-emissions-idUKKCN0VI1C9