I'm sorry, I went back and reread your initial post (my bad, being lazy here) . .
Yeah, I would think ADDING a shim for the described problem is the exact opposite of what I would have thought best. Is it possible that the saddle is bridging? It can be hard to tell, but you can only really tell by running your hand under the tree and feeling if it is sort of 'loose' in the middle portion.If you feel a very mild bridging, you can shim THERE, in the middle portion of the tree. Start by putting a very thin small towel, folded, to add a bit of thinckness to only the middle part. (on top of the wool pad)
Also, for general saddle fit evaluation:
When the saddle is postitioned correctly, can you run your hand down under the front edge of the saddle, or does it feel really tight? Could you photo the saddle on the horse, NO blanket at all, cinched up?
Also, it might be that the saddle will ride better if you use a back cinch (and, don't forget a cinch hobble!)
when the saddle is placed on his back correctly, not cinched up, and you put one hand on the horn, and one on the cantle, can you rock it back and forth? a little, a lot, none?