I made mine out of sheet aluminum from my local hardware store, and some plastic lawn edging from home depot. It attaches to the stock GTI valance with dzus fasteners.
I would wonder if it would be an erodynamic disadvantage... looks like a big brick wall to me... although it is cool
At Pocono the old airdams we had were huge brick walls and they folded under. It looked like if they didn't they would actually give an erodynamic disadvantage. Unfortunatly you and I can't do much with a spoiler as those big old bumpers are not part of the body, thierfor we can basically only go strieght down and that is it.
Here's a link I found that talks about the early history of the GTI. The article mentions that adding a front spoiler to the GTI added 65 pounds of front downforce at 100MPH. Kind of interesting....