In the days of carburators,(have I spelt that right? Hmm, stick to 'carbs.' in future.) a rough inlet manifold was needed to keep air turbulent and petrol droplets in suspension. It was found that with a pollished manifold and cold engine, so much petrol condensed on the manifold walls that the car became undrivable due to the weak mixture. However, with injection, the plenum and upper inlet tracts only carry fresh air, so a bit of a clean up would probably be OK.
What would be good for airflow, would be to match all the ports, ie. make the holes in the plenum the same size and shape as the top of the inlets thereby removing any edge or step. The bottom of the inlets should match the ports in the cylinder heads in the same way.
Depending on how bad things are as standard, you could see a 4 to 8 b.h.p. increase but only at high revs. and large throttle openings.
As for the 'ram air' idea I am not sure. It was a bad idea with carbs. because the carbs. got tuned on a rolling road with the car stationary but when in motion the 'ram air' effect varied the mixture. The amount of variation depended on speed, revs. and throttle position.
With injection measuring air mass and temperature in the intake ducting, it might work.
All the best with your project, Eddie.