My first guitar was a 12-string import from mexico, with a neck about 2.5" wide and a body only slightly smaller than a basso-sexto. It stood up to standard pitch tuning quite well, but that might have been due to the robust construction. Most of the other 12-strings I've known of eventually bent or broke when left at concert pitch. One of the few exceptions I've seen to that is my cheap Ovation, which still seems very stable after maybe 15 years. It took some fiddling with the truss rod and bridge height occasionally to keep the intonation and action reasonable during the first couple of year, but since then I haven't had to change anything. Dropping the tuning a fret or two eases the neck back and lets the strings rattle just a little on the next fret when picking with enthusiasm, its a delightfully dirty sound.