Well I had a quick look to calculate rotor mass removed due to wear, and with my rotors worn approx 1.5mm on both faces (3mm total) it appears that each face has had 318grams removed. So 636gram of material removed per rotor compared with when they were new at 32mm width. It seems really extreme to me. But metal is very dense.
This would mean the worn rotors would have weighed 10.726kg and 10.706kg when new. So the actual weight loss with the new RB 2 piece rotors is actually closer to 1.934kg per rotor. Not the previously mentioned 1.3kg.
This also puts it much closer in line with other measured weight differences between single and 2 piece rotors such as from Subaru and Evo brakes..
However there is also a very interesting video by someone who seems to know how to calculate and weigh things better than me.
He claims that the actual weight saved by going to 2 piece alloy construction hat is only closer to 750grams. He claims the rest is mainly due to different disc rotor designs (the actual rotor and mainly vane design).
I would believe him as I have noted significant differences in weight between difference brand single piece rotors in the past (on other vehicles).
How much weight does a 2-piece brake rotor REALLY save? - YouTube