Are those clips really detachable? Pistons are often cast around a steel core to control expansion. The idea is to cause the piston to be round when it's at operating temperature. This may be a different way to achieve the same result. Balance pads are generally made of thicker aluminum.
Nural | ENGINE PARTS (UK) LTD (bottom of page)
Since you have to balance all pistons to the lightest one, you've got some work to do. 20 grams is something like 7cc of piston material. I'd be studying the pistons to see if I could spot where material is missing from the lightest ones. Any area with a forged surface has the potential to show piston - to - piston variances so I'd measure crown thickness under the pin, distance between pin bosses, thickness of pin bosses, and thickness of skirts. Seven cc seems like a fair amount to me so I'd be looking at everything. How close is the weight of these pistons to the originals? If MB balanced the rotating assembly with original pistons, are you going to re-balance it or just hope for the best?
Interesting reading:
Eaton Balancing » Engine Balancing, Part 3