That is quite a bit of heft. How does it impact the acceleration figures? 0-60 in 6.4 seconds for the rear wheel drive ML350 and 6.7 seconds for the all wheel drive car. Just about equal to the 1/10 per 100 pounds rule of thumb. The 1/4 mile figure for the rear wheel drive ML is 14.9@94 versus 15.1@93 for the 4Matic.
At this power level, the rear wheel drive is the superior performance option. It's lighter, it's more efficient, it's simply faster. The all wheel drive naturally would show its benefit in conditions where surface traction is poor such as snow or rain. Additionally, it will not be until much higher power figures are involved (such as the e63 AMG with its over 550 horsepower) where the all wheel drive will provide much more consistent acceleration performance.
Clearly the ML350 motor does not provide enough power for traction to be the limiting factor in this comparison. When the 4Matic E63 hits expect better 1/4 mile times from the all wheel drive car but better roll on acceleration from the lighter rear wheel drive car.