First, You need to go to a shop that is experienced in current Mercedes Benz transmissions. Don't go to the cheapest one in town.
Someone with the Mercedes Computer can diagnose that trans. properly.
I don't know if there is after market equipment that will read the fill times and fill presures, but those are the key to diagnosing that trans.

Secondly, that transmission did not come with a magnet in the pan. It's OK to add one later, but I've heard stories of people installing magnets of the wrong size and in the wrong place. I think it's plausible that a big enough magnet can pull down the filter or even effect steel check balls inside the valve body. So it needs to be the right size and in the right place.

Bottom line: make sure the shop is experienced in Mercedes transmissions.