I tried it, after a reboot I can select the Hi-Res audio but as soon as I play a track the app prints "Failed: Poweramp Hi-Res Exp Output. Using default output."
My opo (6.0.1) doesn't have the line you mentioned in the build.prop file but only the 16bit one. Does this mean that it is "optimized" out as you mentioned? Would a local.prop identical line help?