Duratec 2.3 and the current ST engines have a balance shaft assembly in the oil pan area, driven off a crankshaft gear.
Ford's recently gone to an aluminum housing for it, previously it was cast iron and the whole thing weighed in at about 20 lbs..
Removing it is fairly easy, 4 bolts plus screwing in an adapter to block the oil passage to it. Saves a bit of rotating mass, adds about a quart of oil capacity, and removes that weight from the front of the car when deleted.
No big gains, you can feel the engine rev a bit quicker in neutral and there's a slight increase in vibration - feel the engine run vs. so smooth you can't tell it's on.