Apart from the GIFs, smoother bearings might produce a tiny improvement. It's not any kind of major variable. What variables improve mileage?

1. Proper tire inflation: Tires that are below car manufacture specification spread and drag. That creates heat, and reduces mileage. Parenthetically, I tend to fill mine about 3-4 psi above the 32 psi recommended by Toyota. That's still approximately 10 psi below the max. The ride is a bit harsher, but it does help a few percentage points, in terms of mileage, IMHO.

2. Proper air/fuel mix: Any obstructions in the air flow (such as a dirty air filter) will dump more fuel into the system. That will reduce mileage. 

2.a. An engine (and related components) that have been abused or neglected will also reduce mpg. 

3. The most significant variable: Driving style. If one has a "lead foot," scoots up to stop lights and puts on the brakes, rather than coasting, zooming from red lights, etc., that's THE MAJOR VARIABLE in the potential fuel economy mix. It can affect mpg as much as in the 10-20 percent range. 

So, forget about the bearings, if maximizing mpg is your goal. 

What, exactly, are you smoking?

https://www.youtube.com/watch?v=BwSts2s4ba4

I agree with @geriscan. It also does not make financial sense.

  I do imagine it might also conversely make it then harder to brake and thereby use up brake pads quicker, perhaps.

Stopping distance would be perhaps 1 cm longer and you would have to change brake pads 5 meters earlier.