The diesel G is even slower than a 240D -- it weighs almost twice as much (6,000 lbs is closer to the truth than 5,000 lbs, not to mention that it's the antithesis of streamlining -- so the result is obvious). I've got a 280GE, and I'm spending a pretty penny to get more horsepower (see my post on this forum).
Don't get me wrong, it's got planty of power at lower speeds when you can use the gears -- it's at highway speeds (on hills) when you'll feel the shortages.