Before you guys get crazy, I would have the manifolds flow tested first, and then flow test the exhaust part of the heads. This will tell you if you really need headers or not!
The AMG SLK32 has ported heads, so I'm willing to believe the heads are more restrictive than the exhaust manifolds. A bigger exhaust valve would be a cheap answer to porting, the 2 intake valves are already at 100% efficiency, but that 1 exhaust valve is not. Don't believe me check it out yourself, any SAE book will tell you this.