Optimal digital cable length -- science or voodoo?

1.5m might be optimal for particular slew rate of the output driver but it is different from transport to transport. Some drivers keep constant dv/dt (slew rate control) while other don't. Calculation of the reflections effect on the signal shape is pretty difficult since output of the driver is nonlinear. Long time ago people used Bergeron diagrams to evaluate reflections in transmission lines but it isn't simple.

If you hear difference between 1m and 1.5m cables then modding (or at least evaluating) output driver on the transport is probably beneficial.

Steve - I feel the same. 500GB external Firewire drive costs about $100 and can store around 1000 CDs. Second external drive can be used as a backup kept in different house. In addition other formats with more bits and higher sampling rate can be supported. All this and convienience.