Passive Air Restrictor?
 

Is this injection molded structure (red) in front of the air intake a passive air restrictor?

If I remove a section of it, would horsepower increase (theoretically) or would the ECU compensate for the change in intake and performance remains the same.

Possibly a flow director to help ram air. MB would not want to spend money to install a part to reduce power and efficiency. If they could do without it they would. Restricting air flow by the addition of more parts at an added cost has never been a method to improve gas millage.

I thought it was to prevent rain water getting in to the intake.

Ding ding ding, we have a winner!

That geometry is definitely meant to separate water from intake air.

Also, it's not even close to the only inlet path for intake air, so you shouldn't think in terms of "ALL of my engine's air has to get through these ten tiny slots".

There's nothing to be gained by removing material from that area, and there's a lot of potential harm to be caused. Put down the Dremel.

Google Image search for "hydrolock piston", you'll see why water separation is kind of a good thing to have.