I don't remember where I read it, but, I read reviews on a fiberglass Ghia made in England a few years ago.  I don't know if this is the same company, but the article I read about them said without a magnet, they were hard to tell from an original metal body.

The main thing I would wonder about is the body and chassis flexing.  The coupe's roof was the main thing that stiffened it and the convertibles had an extra metal beam inside the rockers to keep it from flexing.  Without some strong, stiff metal running down the outside of the pan, I would think that the fiberglass on the rockers would be cracking within few months.