The 55 is nowhere near being a super car.
The performance figures aren't good enough in todays market for that label to apply - but even if they were, it takes more than just performance to warrant that tag. It lacks the theatre and visual rarity - not least because it's just a fast version of a relatively common car.
You nailed the response from my point of view. I would only add that the definition of "Super Car" is constantly shifting as technology advances.
Maybe the SLK55 when it was first introduced in 05 or 06 was close
to super car numbers of that day in a couple of measurements. But it has never really achieved that status. And that was never what Mercedes/AMG intended for the car. Too many produced (exclusivity), never at the pinnacle of the numbers game (performance), no really exotic materials or technology (design), just tried and true engineering.
For me, what the SLK55 represents is sort of the German interpretation of the U.S. pony car. Big engine in a small chassis, two doors, great numbers if not the very pinnacle, at a cost far below the "super cars", in a package you can enjoy every day. Mercedes just took it to a completely different level with a folding hard top and a level of refinement that U.S. pony cars will never achieve.
If I were going to call the SLK55 a "super car" it would be because of the overall value + enjoyment + performance the car delivers in the real world every day in a pretty durable package at a not outrageous price. That's the stunning achievement of this car in my book. 4.3 seconds 0-60, six piston brakes, 400 hp blowdryer, and the quiet of a sedan, nav, Harman Kardon audio, and digital climate control. There aren't many "Super Cars" you'd want to live with on a daily basis over the long haul.