Don't know what the numbers are officially, but a lot of people online are suggesting around 4kg of weight savings. For the same money that Porsche is charging you could probably cut more with an aftermarket carbon fiber hood.
Forgot where I read it but basically the savings are so little that the price to upgrade can be a massive turn off.
Being a big part of motorsports culture, its easy to see why some people will get it regardless. Heck, even I would, just one more way to distinguish your 992 from the other guy.