Giving better conductivity, therefore better performance,
Not exactly. Conductivity is not so critical. Better boards have shorter traces and consistent impedance. One way to make shorter traces is to interconnect parts exactly as defined by the IC manufacturers.
Thickness of copper is not important. Other factors such as consistent distances between layers are critical. For example, if one copper trace passes over a 'hole' in the ground plane, then that inconsistency causes the trace to act as an antenna; causes signal anomalies.
Also critical is positions of numerous bypass capacitors.
Signal currents between ICs are typically so low that no copper trace can be too thin. Resistance is not relevant. But impedance is critical. Impedance is not determined by copper thickness. It is determined by other parameters such as sharp bends, trace length, and a relationship to other layers.
