I know that there has been lots of speculation in the past so I weighed my S2 GSL rear and my S3 GSLSE rear. The results are suprising and likely some will call BS but the bathroom scale tells no lies.
S2 complete with everything including gear lube and park brake cables; 140Lbs.
S3 complete minus park brake cables and no gear lube; 142 Lbs.

I'll have an S1 drum Brake rear on the scales in about a month and I'll add it then.

 

just wanted to add that the 8.8 rear that many use is 250lbs

 

So it'll be 150 pounds fully loaded for the S3 -SE rear. Larger rotors + larger calipers will do that. From the last time I was messing around with rear ends, the GSL gave up ~10 pounds to the drum brake rear end (both full of oil, both with their brakes in tact...)