I had a Fibreglass one on my old car, they look ok, but you dont get the cut out in the spoiler as they cant make it with the gap, the cut out I'm talking about is the one you can see when you look at the car from behind, the gap between the top and bottom of the spoiler, also the one I had didnt have the correct curve for the bootlid so was a bit of a pig to fit.

I know the spoiler is heavy but I would keep it, see if you can get a fibreglass tailgate instead, and maybe some perspex windows to help with weight