hi,
Curious, I bought a carbon fiber tub today. Uncalibrated scale...
plastic tub around 155gm
carbon fiber tub around 150gm
only around 5 to 7 gm difference. carbon fiber also has more holes... if you made the same holes with the plastic tub you might end up with same weight.
carbon fiber tub much stiffer, especially front to back bending. as to which has the tweak, i could not tell.