what is the weight difference between the gta rubber spoiler and the fiberglass aftermarket ones like the 4 gen style? Will this help my sagging rear?

 

I don't know about that, but the diff between the '82-'90 wrap around and the '91-'92 wrap around is about 40 lbs. The late model is ABS plastic.

 

I've heard the factory rubber wrap-around weights close to 50 lbs! Although I have no clue how much an aftermarket fiberglass one would weigh. I'd guess there's a 30-40 lb difference though. As far as the sagging rearend/springs, I can tell a difference on my car between when I do and don't have my toolbox in the back. It's about 70 lbs though. But it can't hurt. Besides, if your rubber spoiler isnt cracked yet, it will be soon.