Do I think Mercedes left a bunch of Hp on the Table? Hell yes I do.
All you have to do is look at the specific output of the current BMW M3 or S2000 Honda, which did sacrifice some low rpm power for top end, but also has a 120Hp per liter output. I could live with a little bit less low end for performance like that.
Lets be real, these newer 3 valve motors are not works of engineering greatness with regard to Hp. There prime emphisis was good emissions and respectable Hp.
100Hp per liter is not leaving Hp on the table. 70Hp per liter is not really earth shattering Hp for the C43. So there is lots of room for improvement.
In addition the car ran very hard with the new exhaust. The loss in response started a couple of weeks after the install.