The thing is, 1/2" leaves very little room for error. If the glass has a small defect or it was not handled well, if the bracing is sub par, or if there is above average stress on the glass, you have an easier chance of a failure at 12.5mm, then at 15mm.
You could drop the height to 24" (not 27") and have a much safer tank, but that's a personal call. In a garage or basement, for someone that doesn't travel and doesn't own expensive fish, it may mean one thing. In a living room with expensive flooring and electronics for some one that travels and has expensive fish it might mean something else.
I've seen minimal water damage up close (from fireman hosing down my apartment.) It's a disaster that I hope never to repeat.