Superchips U.K. has developed a firmware upgrade to boost the power of BMW's 4.8-liter V8. The mod does not require any new hardware - technicians reprogram the car's ECU through its diagnostic port. Within an hour, your BMW 650Ci, 750i or X5 can have 386 hp instead of the stock 368 hp (see dyno charts above).
Cost for the upgrade is approximately $450, or roughly $25 per hp. Superchips has conversions for a host of cars and engines, and a worldwide network of dealers. The U.S. Superchips site is here. Be aware that there's no free lunch - horsepower gains usually require a change to premium gas.