I have installed the Eastern Beaver PC8, too. Yes it does allow me create an 12V negative ground bus bar, just like you must in boats, but it also has other benefits. The PC8 has relay driven and an unswitched supplies and an earth cable going through dedicated pure copper wiring back to the battery terminals. This allows no extra physical connections to the battery posts making them less of a weak point in the electrical system and also does not allow galvanic action with other inaccessible components on the engine/frame. Copper wiring has minimal voltage loss. All of my accessories are therefore connected to the PC8 which makes adding, deleting or amending them easy and accessible.
In addition, because the power distribution is at the PC8, so are the respective accessory fuses; otherwise all accessories at the front of the bike must necessarily be fused and/or earthed inside the nose to protect wiring which can be a a PITA if you have problems. By having a separate ground return for accessories, including audio, you will minimise interference.
I take the attitude that the bike's wiring is designed to only handle what the bike is designed by Honda to do; any thing you add must have its own wiring to support it. Therefore the only electrical common components are the battery, alternator and main positive and earth tails (assuming the battery terminals, alternator terminals and tails can handle the current). The alternator is rated as 740W which at 13.5V output gives a current of about 55A at 5,000 rpm starter capacity is 100A at 12V so I would reckon that the positive and earth tails are deliberately over spec'd and will take anything you throw at them.