Title says it all…
If it is a good supportive pad, like needed for treeless, with good foam and spinal clearance, I think the fact that it is a western cut wouldn’t matter.
I love the Reinsman thick wool felt ones - if you don’t mind the odd look when paired with an English saddle.
Which treeless are you riding in? I’m searching since the saddles I have now destroy my hips after every ride - even basic ones. (Stephan Peters and M. Toulouse - both from Dover).
Well, off the shelf tack store saddles in the Lower price ranges are rarely the best choice for anyone.
If you are having pain issues I suggest finding an independent saddle fitter and a massage therapist or chiro or physiotherapist.
Figure out what kind of body you have, length of femur, pelvis, etc and if you have any asymmetries like scoliosis or hip angles.
You may need a narrower or wider twist, a larger or smaller seat, different stirrup attachments, or some stretches and exercises at home.
You might be much better off with a second hand quality saddle that fits you than something off the shelf.
In geberal I think people with hip problems may do better in a narrow twist on a horse with less bulk. Treeless saddles tend to have a wide twist and if they need a thick pad that adds to the twist.
How does it feel to ride this horse bareback?