I came across two reviews of Brooks Cambium by a guy who says he has 105-110 mm sit bone width:
https://www.foldingtales.com/home/20...-saddle-review
https://www.foldingtales.com/home/20...-saddle-review
He does performance (not upright) riding, and he likes 132mm wide C13 better than 140mm wide C15 (though C15 has no problems either).
That probably means that for someone with a 130mm seat bone width C17 should be fine for performance (not totally upright) riding, there is no need to go for C19. Will try C17 and report.