It would depend on a couple of things.
First, how well BMW engineered the car. They SHOULD have made the ground wires large enough and made them contact well.
Second, the connection points. If they are dirty, then certainly cleaning them would increase grounding capacity and help.
Third, the age of the wires. The first two might be fine, but if the wires are cracking then you might see gains when replacing them.
You might just do it to clean up old wires and make your engine bay look better by grabbing colored wire and lowering the gauge one step. Unless someone else will chime in to actual results, Im pretty sure I got all the points. Using copper wire, copper connectors, and conductive grease would be prudent to ensure high conductivity and protection against corrosion. | 
Grounding Kits 
Linear Mode
