You can already get fibreglass wings for a mk3, have seen them on a few cars.
However, people need to remember that you are taking away a lot of strength from the front end, the wings are an integral part of the crash structure, not sure where you stand with the mot as well as I think they did used to be classed as a failure.
As for weight saving, with the extra material needed to make fibreglass panels strong enough to use they are not a lot lighter than steel.