I have the 2018 ergo fit oem seat on my 2015 and it fits. The reach is noticeable because of the thinner nose on the west. Made me buy some motowerk footpeg because of the low seat.
I have a 2016 model and i have just been in touch with my local kawasaki dealer to enquire about the ergo-fit seat, the seat is listed as fitting a 2017 model, he looked at the standard seats part numbers for 2015 and 2017 and confirmed they are different part numbers and said it may not fit.
After the call i gave it some thought and viewed the fiches and both seats look identical, so i think the part numbers are different between the two models because of the different foam construction on the 2017 models and above. And the seat base is quite likely to be the same.
I have emailed Kawasaki head office to see what they say about it.
Can you confirm that your ergo-fit for the 2017 and above installed just like the 2016 model without issues or modification/.