Hi Barebow, I've got some experience with ipe, and I pretty much agree with all the above, I 've experimented with different widths and I concluded 1 3/16" was best but I also made wider and flatter and that was good too, better for if you recurve the limbs because they are more stable, 1 3/8" is plenty for this, but like Rich said dont cut it all at once, cut one stave and see how it goes. Use a high angle plane if you are going to plane it, or put a high angled new edge on the blade you got otherwise planning can be difficult.
also I dont think ash is strong enough to compress the ipe and isnt suitable for a backing you need something stronger