Transmission speed is going to be dependent on the conducting material, and correct me if I a wrong, Cat levels are bandwidth capabilities enabled by the reliability of the signal, therefore the shielding around each core. The transmitter/receivers at each end are going to have a significant effect on this.
Optic fibre is good for data and low level audio. I believe @geoffkait stated that jitter was a debilitating effect on music over OF. I would imagine that this to will be overcome in the future.