Hartmut Feyhl left AMG after 15 years to start his own tuning company, RennTech, which is now one of the most respected AMG tuners in the world. His products range from simple software upgrades to full racing builds based on AMG street cars. We take his new, 700 horsepower CLS63 for a spin and establish a new benchmark for luxury sedans: must be able to do burnouts while moving at highway speeds.