the clk 55 is a lovely car , i always wanted a convertible version . The Autobox is fine providing you cool it correctly. I have all the nessary coolers on the car. I have rebuilt various boxes and they have with stood 600hp without any issues . People dont understand that biggest cause of autobox faults is down to heat. The car has been completely rebuilt as you can see from the spec, yes if you wish for more HP, you would need to get it re-tuned. The setup is good for 600hp, its got all the items required for this. The only thing i would look at putting on is a oil cooler , the stock unit is great , but i rather be on the safe side.
give me a call if you want to have a chat, pm me.