Effectiveness is dependent on the stock wiring system design. One thing to keep in mind in Scions in general and xB2s in particular: they are designed to be modified. So, some stuff is actually overkill for stock. The brakes are designed to provide a "normal" feel for 20" rims, so they are really more than needed in stock form (and make no need for the "big brake kit" when running light rims). In fact, the reason stock steelies are 16" is that those are the smallest rims that fit over the stock brakes. The suspension also "upgrades" very easily too. As rims and stereos are the 2 primary mod-areas for Scions, I'd be surprised if overkill grounding wires weren't used originally- thus allowing for amps and so forth to be well supported w/o extra grounding wires or a larger alternator.
