In reply to ablackett:
imo there is a shocking lack of understanding of gcse level physics on this thread.
acceleration d/t gravity = gM/r^2 where M is the mass of the Earth. Therefore to achieve any additional acceleration down the hill you'd have to change the density of the Earth (and hence the term M/r^2) . This is actually possible due to local inconsistencies in density (https://physics.info/falling/grace.jpg) but you'd have to move the race.
It has been suggested that this law only holds in a vacuum. Drag does indeed retard the force of gravity by
Fdrag=p(v^2)CA/2 where p is the density of the air, v is the velocity of froome, C is a dimensionless constant and A cross sectional area. Therefore, as changing the weight of the bidon has absolutely no effect on the drag, and consequently does not effect acceleration due to gravity.
In the turns however, the physics is complicated, gravity, inertia, traction and balance s involved. Its described (incompletely) here https://physics.info/falling/grace.jpg but the amount of understeer needed depends on the height of the centre of gravity. I wonder then whether lowering the centre of gravity with a lead bidon would lessen or enlarge the amount of understeer needed for the perfect corner line. It might well explain why moving the bidon up to a jersey pocket has an (apparently positive) effect on cornering speed.