Search for Eisenmann exhaust system. Few aftermarket exhaust systems actually significantly improve power in the S-class (because the stock system has pretty good flow). If anything, most people want to go for the throaty sound. From what I've heard, the Eisenmann system sounds like heaven and will save you a lot from that Renntech system.
If don't wish to spend a lot (like me), you could cut off the resonators and weld in straight pipes ($50-$70 for everything), or cut off the mufflers and weld in aftermarkets (like Magnaflow, which is my plan, about $250 for everything). They only alter exhaust note though.