In this case, I would not use butters in the bar. I am generally anti saponified butters, but in this case I think it will still cause more trouble than it is worth.
If you want to use them up, make a lotion or body butter or something, but in a soap specifically for skin that you don't want to have any additional friction than is required, keep them out. The one with 30% butters might not have so much drag that it is noticable in normal use, but on this skin it might be a different story. Even 20% could anger the skin more than it helps.