Electrical systems work by moving electrons from a higher voltage point to a lower voltage point. Current is how fast it is moving. When the ground is not always 0V, the current will be lower. Hence electrical systems won't work at peak specifications. Electrical systems such as your fuel injectors. Fuel injectors won't wait for the perfect current. When fuel is required, it is sprayed. If the current is lower, then the injector spray pressure will be lower, hence less atomisation. It's only a small difference. You can feel that it is smoother, but I don't think you can feel that better grounding has made your engine produce more power.
I'm more interested in the capacitors. I hate it when my lights dim. I have been told that higher specced batteries fix the dimming problem. Optima batteries.... drool..
