My distance mare (Half-Arabian/Appaloosa) is big shouldered, not high withered or short backed but hollowed behind the shoulder and slab sided - meaning that she tapers from being wide at the shoulders and narrow through the rib cage.

We ride her with a cutback Passier hunt saddle, specialty pads (removable pockets) and a breast collar for distance and showing. Her Circle Y Equitation western saddle seems to fit fine with no rubs or sliding backwards.

Muscling her up with the proper frame did correct some of the structural problem as it pertained to muscle groups.

YES YES and YES. I have two arabians like that and have found some happiness with the fit of my treeless saddles. I currently have the Barefoot London Dressage and love it with a no-slip physio pad and a wintec neoprene girth. I also have a Torsion Treeless that fits very well but I am not used to the way I feel in it yet.

Yes, I have an 11 year old Arab gelding. I've had him less than a year and with work and feeding his hollows and dippy back are better filled in. I found the Barefoot saddle fit him regardless of seasonal weight and shape changes and he is less cranky about saddling. I also find the Sensation saddle (also treeless) fits him well and is even more comfy for me than the Barefoot, although they are both good. Sometimes bad saddle fit causes them pain from bridging or pinching and then they move badly and muscles develop poorly. A good quality treeless can help with all these issues (skip the ebay knock offs, but a good quality second hand one can save your wallet).