I have an Abetta Endurance saddle and it is all "pleather" on the pommel and such (no horn, so no leather there). It has a couple of leather pieces on the "conchos" and under the fenders. You could replace the conchos, but the fenders really need some reinforcement where the adjustment holes are. Without something sewn in there, you risk the nylon unraveling.
It's tough to find a 100% leather-free saddle. Even Wintec English saddles use leather billets, or at least they did the last time I bought one (2005).
Circle P saddles are Asian made junk. The only synthetic western brands I would consider are Abetta, Fabtron, Big Horn, and Wintec, in that order.