I think the G20 is 121lbs lighter than the F30:
https://jalopnik.com/the-bmw-m340i-d...-ma-1830405871
The F90 M5 is 45lbs lighter than the F10 M5
even with AWD.
https://www.roadandtrack.com/new-car...6/2018-bmw-m5/
The 992 is heavier mostly because the entire car has been engineered to accept a hybrid powertrain.
Most likely, if a similar fate were to befall the G80, we'd have seen some weight gain in the base G20.
I don't think we'll see the same 121lbs savings that the G20 got, since the F80 already has some of the aluminum parts that enabled the G20 to get that low. But I also don't expect a huge weight gain over the current car.