I just changed my 990 to a single rotor. Here are the actual weights...
This is the two calipers. So you save half of that which is 1.13 Kg
This is one stock rotor. 1.38 Kg
If you go with the Galfer rotor you save an additional 0.5 Kg
So total removed unsprung weight is 3Kg or 6.6 lbs
You decide if you think thats worth it.