The SLK55 AMG hits a respectable 362 wheel horsepower stock on a dynojet. That is a peak figure and the majority of the runs were in the ~350 range. For a rated horsepower output of 415 the SLK55 puts out good power to the wheels and may be just slightly overrated. Another way to look at it is that the motor dyno's on average a few horses higher than a stock E92 M3 despite being rated only one horse higher and having an automatic transmission.
With this motor only being utilized in one car currently it is unknown if it will get good aftermarket support with such a comparatively small market. There is potential it looks like for over 400 wheel horsepower with bolt on's. Not anywhere near as potent as an M156, but respectable.