It is not additional pressure that you need it is volume. As the car approaches red line the volume of fuel from the factory pump drops off to around 45- 50 gal/hr this allows the rail to start to empty. The SL55 pump works under the same pressure as ours but can supply 95 gal/hr @ idle and 70-75 gal/hr at redline this way the rails don't start to empty. The SL-55 cars run 8 cylinders and dyno over 600 at the wheels with the stock pump. SLK32 Germany uses this pump and runs stacked pulleys with no lean conditions.