I did some more inspection of the R9 11S carbon-pulley RD to figure out why it's so freaking heavy.
My notes:
-R9 (and I'm guessing RX) aluminum material is cast aluminum. You can see circular marks on the parts where the aluminum was poured into the cast. Dura-Ace, Ultegra, Force and Red RDs are machined from billet. Casting of course is much cheaper, but the reasons why it makes the derailleur heavy are threefold: 1) billet is almost always a stronger material per unit of weight than cast, so therefore the theoretical limit of how light the part can be is lower. 2) complex and thin features that save on material and weight are harder to cast, so the part is bulky in the first place to make casting possible. 3) It's not really possible to machine cast aluminum to make it lighter in a manufacturing setting. I do think I'm going to attempt doing some derailleur tuning with my dremel though anyways
-Hanger screw appears to be chromed steel on the R9, while on the RX it looks like aluminum. This probably accounts for a lot of the weight savings on the RX
-Cable screw is not tapped into the material, but rather backed by a thick steel nut! I think there is a chance to save a gram or so there alone.
-Lastly, I've identified the play in parallelogram comes from the width of the parallelogram links being slightly smaller than the body, which accounts for the play in the parallelogram.
In general, this rear derailleur is designed to be dirt cheap. You are not getting anything that approaches Force or Ultegra quality with these.