I'd be surprised if distance over 1400m has too much effect, my guess/assumption would be that you generally find larger fields over that distance which would be the contributing factor to favourites winning less, not the distance itself.

Happy to stand corrected though.

What would be of interest is the following tests although it would be time consuming:

Favourite stats at 1400m based on size of field (i.e. 7,8,9,10,11,12,13,14 and 14+ runners)

Then do the same for 1600m and so on, I've suggested 14+ because i've found myself doing my ratings that 8-10 is the sweet spot for picking winners with ratings, 10-12 offers some value, 12-14 it gets quite difficult and 14+ poses to much potential for issues out of the jockey/trainers/horses control.