The car I saw was an e30 318is. His before and after weight was more than that. But he did some really thin stuff for the rear and back rear windows.
Layed it against the old glass and cut the pattern. Then cut the tape in about an inch all the way around and painted it black. Layed it into position and started at it with a riveter. Then he had two thin aluminum bars running down the middle about 20 inches apart to keep the back from flexing. Just a little added strength.
You can DIY it, and save weight for cheap. More economical then trying to get power out of an m42..
The only reason I don't want to buy something like a sparco or OMP is because then I would have to buy two to make them match.. Part of me says screw it, doesn't need to match. At the same time I think it looks so much better.
I wanted to drive it last night so I put the driver seat back in after finishing up with the sound deadening.