I think the weight thing is just as likely some teams wanting more robust cars, but at the same time possible sandbagging big time. In past years if we believe testing times we might think the Merc is off the pace, but we know better. Some teams might be well under weight, and just play along hoping it gives the other teams false hope and/or so they have better ballast options.
I doubt even the hands on crew knows the specifics, other than the guesstimate of the weight they feel in their hands at times. The setup and such requires some basics of overall mass and COG, but not much info on the specifics of each part.
And at this point, we know all kinds of new bits will show up within a few days now.