I finally got a few minutes to kill at work and took a tape measure into the warehouse. The SLK 230 has the same size brakes as the Limited. Nothing to be gained there. The SLK 320 has the bigger discs. Obviously, the SLK 32 has the good stuff.

Those monster E55 brakes are a wee bit heavy. The calipers really don't weigh much more than a smaller cast iron or steel caliper. The aluminum really helps. However, the rotors are 27 lbs each. That's ugly. The flip side is that you've got MUCH more braking force to slow down the rotating weight.
Should you be working on a purpose built car with custom tailored suspension, the unsprung weight will cause you to rethink your spring and damper rates.