Originally Posted by PaPeRo

It doesn't matter if the shaft itself does not rotate, it doesn't need to. From the POV of the ball bearing it's the same thing. If you hold the center sleeve of the bearing so that it doesn't spin the bearing still spins. That's the whole point of having a ball bearing. If the carbon shaft doesn't need to rotate then the steel one doesn't either.

As for weight that depends on what you're looking to achieve. Physics says lighter weight = less mass = less inertia = better handling IF you know how to set it up with the right tires etc. If more weight equals better handling we'd all be driving dump trucks.

Also less weight equals higher acceleration again assuming you have the right tire and suspension setup.

What you want is better weight distribution, not weight.