There's really not much to do to your car other than supercharging. Underdrive pulleys will get you a small gain but not really enough to make them worth the money.
I'm not trying to restart the debate, but it looks like they both have their advantages (and disadvantages), assuming this info isn't biased. I guess it all comes down to your needs, preferences, and budget...
I too am thinking about upgrading to a supercharger on the 55, but I have a question for both HPS and HOP...why can't you provide the same HP gains as Kleemann? Right now Kleemann seems to be in the drivers seat for my dollars as they can put 600+ in the CLK55. I would be happy with even 550HP. Will that be a reality in the near future? Thanks for any info you can provide