I think it would be an upgrade to install a secondary independent trans cooler, either in-line with the existing one, or as a replacement.
Granted, it is not as much of an upgrade then if you install a trans cooler on a car that does not have one to begin with.
Now, I am sure Mercedes-Benz properly sized the radiator and transmission coolers sizes when they designed the car, but we all know that as radiators\coolers age, they begin to loose their maximum cooling capacity. due to a number of factors.
Since the trans cooler is built in the engine radiator, it has to share a bit of the heat built up with the engine coolant. By installing an isolated unit, you eliminate a huge amount of this shared heat.
Options:
1. Leave the cooling components alone, put in a rebuilt unit trans with new cooler lines.
2. Source either two small units to be placed near the FL and FR of the car, or one larger unit to fit near the radiator/AC condenser. (Custom mount brackets would be needed.)
3. Part out the car!
I think Option 2 would be really neat. You could have the hot fluid from the trans enter an small isolated unit near the FR of the car (assuming that is the closest unit to the trans oil supply line). Then after going through that unit, it could go through the stock section of the radiator. After it passes through the stock section, it enters another small unit near the FL of the car before coming back to the transmission.
While you are in there, you could also install a temperature sending unit in-line at both the entrance to the system before the FR unit and at the exit of the system after the FL unit. Hook up some gauges and see just how effective your new system is!
FL = Front Left
FR = Front Right