Like Mattnew, I have owned an Ultimate for the last 20 years. Actually 3 different ones. I fly mine HARD. I have never had an issue with the outer cabanes. I have done a blender hard enough to rip the center alum cabane mount out of the ply in the fuse. (it was a thing a beauty but should not be done with a plane you like) Even that did not hurt the tabs in the wing or the outer cabane. So please use the 1/8" and save your self some weight :-)
David