MBworld.org has some amazing knowledge. It is a little deep though to dig through, and it isn't always the most intuitively laid out. Here are two of my favorite threads on MBworld.
Build the most powerful CLS 550 4matic is one of my favorite threads on any forum hands down. First this dude decides to build a 550 instead of a 63. That right there takes some balls. Then he upgrades the CLS to 63 turbos and tests it. Decides it is too restrictive and builds his own setup. The entire time this Ol' G is documenting the restrictions, and his gains. Pushing the knowledge of the entire platform forward.
Hands down thanks G!
Brabus_TX puts together a two threads about Mods and their efficacy. He cultivates a group of like minded people who push the e40 tuning to the m157 platform and the gains are awesome. He puts together a spreadsheet of some of the best cars and notes their gains in an epic sequel to the first thread. This guy is fucking Rambo of MBworld, cutting through haters and in first blood and coming back to rescue the rest of us who were captured by gasoline and not really knowing the best mods without a such a meta analysis performed in the go fast thread part II.
Brabus_TX to the modding haters.