The "bussbar" would be for electronic devices that NEED a "clean" ground free from noise. And that same bussbar would have its own dedicated cable to the battery. The whole idea is to separate/isolate the high tech digital devices away from the old school analog devices like alternators, wiper motors, headlights and such. Motors and alternators create electromagnetic fields that cause noise.

When it comes to the starter motor......it is the highest current drawing device in the vehicle. Of course it needs just as high current carrying a path as the positive cable that feeds the starter (A basic law of DC voltage). One little ground strap is not going to do it. If the motor is solid mounted, it should be fine.

High current devices like, headlights, starter motors, wiper motors, when poorly connected will seek the path of least resistance creating a "groundshift" or a voltage drop that causes more problems for sophisticated devices like electronic modules.

Bottomline.......run a separate ground for your ignition modules/boxes and otherwise the use of the rollcage is perfectly acceptable for the rest of the old school automotive circuits.

And the electron flow is within the metal of the wire (copper being best) but the electromagnetic field travels along its outer layer.