Thanks...

The idea
marginal gains through controlling excess fluid movement inside water bottles on the bike

Purpose
reduce incidental fluid accelerations and momentum caused by fluid (in some cases 1 and half kilo worth) rocking in all directions but forward - that could detract from the bikes overall forward momentum

Potential Result
- conserving forward momentum when biking
- making the bike feel less sluggish - there is no rebound time for the water vector to move forward again - like compression clothes preventing muscle rebound
- better handling during braking and steering (in the case of a half kilo or more of water sloshing forward on a horizontal BTA system or horizontal frame reservoir)

The question was
If it made sense
How much could be saved

There have been some great answers! thanks to all

You should probably go back to basic physics to prove that water movement inside a bottle is actually a bad thing. You may find (I think you will find) that it is actually a good thing from the perspective of conservation of energy. You are focused on movement of the water relative to the bottle & bike. You should be thinking about movement of the water relative to the earth. The less it is baffled, the less it moves relative to earth. The less the water moves relative to earth, the less energy is needed to move the system as a whole.

Second, you should probably reconsider the order of magnitude. A full bottle would have very little of this movement, with or without a baffle. So, a typical BTA or frame bottle with 700g of water would have very little movement when full. Conversely, a nearly empty bottle (say ¼ full), would have ample room for movement, but the mass would be 175g. Therefore, your concern over potentially “1 and a half kilo” should probably be reduced by an order of magnitude.

Create a test rig that compares the energy required to oscillate a bottle at ~3Hz. Then, test a 1/2-filled bottle with frozen water (perfect baffle) and thawed water (normal). You could try it at different volumes just to compare how volume factors in the measurements.

This sounds like a great analysis aswell

So if true....does a aero bottle act like a baffle compared to a round BTA with space for fluid to float?