Yeah, I was thinking about the Wintecs interchangeable gullet system. Are they the only ones who have that because they are synthetic?
Wintec is the main first leading brand that carried the changeable gullet system, but it has spread to a wider range of brands. Example; Bates (which is made by Wintec), Pessoa, M. Toulouse (adjustable tree. The one I'm getting), and a couple others. Those I listed above are all quality leather. So, no. It's not because they are synthetic.
Now, the Wintec gullet system, is just bars placed accordingly to the horses size. If your horse happends to be somewhere in between the sizes, then you just have to heat and hammer the bar to the size needed. (I have gone through this). The reason I'm getting the Toulouse, is because the tree adjusts millimeter by millimeter. Much easier to adjust.
As far as fit, as has been said, just make some cut outs or something and take it in, to try up against some different saddles.
Saddles are like shoes. One brands medium tree, is anothers wide tree. You just have to try a couple and see what works best.