It's so easy to gain/lose HP with a little tweak of the ECU. I wouldn't be one little bit surprised if MB decided that the "Chrysler variant" of their SLK simply would not be allowed to make more HP so they detuned it ever so slightly.

In 2004 Chrysler introduced a lower HP version of the PT Cruiser Turbo. There is no difference whatsoever between the HO GT engine and the Turbo "Lite" of the Limited Edition cars. Same internals, same intercooler, same turbo, same fuel injectors, same intake, same everything. The ECU programming in the GT gives more boost and different timing, is tuned to run on premium fuel and allows 220 HP out of the engine. The ECU in the "Lite" drops max boost, retards timing, is tuned to run on regular and makes but 180 HP. With either engine the Stage 1 upgrade is simply an ECU swap and it takes both versions up to 240 HP. I don't see why DCX wouldn't have simply done the same sort of thing with the SRT6.