Couldn't find the forum thread that discusses max HP (as a side issue), but here are two good points

The Crossfire N/A is the highest horsepower Chrysler to use this six-speed manual (NSG-370), although it's used in the Jeep Wrangler and Liberty with 4.0 litre V-6 with more torque. The torque limit is 272 foot/lbs. More here.

Gotbroken lists the 2004 SLK 350 (which puts out 272 horses) being paired with a standard six speed transmission (no indication whether it's the NSG-370). I would probably go to 300 myself but no further. Even then. I would beef up the clutch as far as it would go, get the best fluid possible and pull fluid samples for testing by a reliable lab frequently. Also, limit the bad boy action as much as feasible