Sorry to jump in, not being Pinto Pony... But have you looked into re-flocking your Wintec 500 to make it more banana shaped? My old wintec (11 years) is very flat because fo the wear, but it suits my flat backed ASH. But I just bought a new one, and it is super banana-ey, which makes it a good fit for my short back arab.
Just a thought :]
I have an old Stubben jumping saddle, and it seems to be moderately banana-ey (need a new word!), I wouldn't say it is extreme, but they may have changed the design.