If you were able to order code 357 with your Audio 20 System then it is very likely that you only need the SD card.

At least in the C-Class (not CLA) you can't order 357 without 522. I don't think there is a real technical difference between Audio20 with or without touchpad.
It's just the typical Mercedes Benz money making policy.

I think they will make Garmin Map Pilot the new standard, they started with C-Class / V-Class now also coming in the 2015 E-Class.

Very likely the SD cards will work for any of the new generation Audio 20.

BTW, I just checked if you select Garmin for the CLA you also need code 522 but it is only called Audio 20CD including Garmin preparation.
The Garmin System itself is called 357 as well. So I guess it's the same SD. But you have to have the new Audio 20.