Those ceramic bearings were impressive on the spin test. You can almost use them to balance those rims/tires as you can see the heavy part hit the bottom when the tire slowed to a stop.
It's definitely not going to make much of a difference to me at my skill level. I think your example of battery life is a good one; there are also race classes with limits on motor and gearing that can make little efficiency gains here and there useful.
RX28 uses 3x6x2.5 bearings throughout. Depending on the version of the ball diff you have (i.e: v2 uses a thrust bearing), you need either 7 to 8 bearings. 2 at each of the 2 front rims. 2 on the motor mount. 1 on the outside of the right rear rim. If you have the v1 ball diff, you can consider replacing that bearing on the diff. You don't have to use a good bearing on the outside of the left rear rim because it doesn't rotate relative to the axle.
