I’m finishing up my Morimoto ballast install and had a fitment issue with the driver side ballast. In order to fit the ballast, I removed the Duct A inlet (shown in the attached image marked with a red circle).

I’ve also attached an image of how the air flows through the air intake system (the Duct A inlet is shown in this image with a red X). To my mind, it seems like the missing piece optimizes flow, but is not critical.

I don’t mind losing some horsepower but don’t want to do anything that would risk the reliability of the car. Any thoughts?

 

Looking at the second informative graphic, it appears the 'blue arrow' ingress of cooler outside bumper air flow is just channeled upward, via inlet duct A, into & around the air intake tube C for dilution of the warmer air within the engine compartment?
As a result, the warmer 'orange arrow' air is then actually ingested, via intake tube C, down & around up to the filter box for the engine.
Therefore, IMHO, removal of inlet duct A, to facilitate fitment of your ballast install would just result in a more mixed warmer air charge for intake tube C.
In addition, I'm not seeing a potential for water ingestion since the location of intake tube C is so far upward.
That is, unless one finds themself nose first in a river or flood waters rise to tube C's level.

 

It's funny you mention water ingestion. While I was struggling to find a way to keep the Duct A inlet tube, I was able to fit the tube, but only by changing its position. The new position changed the angle of its opening from flat to the ground to a slight funnel effect and my first thought was, this is going to capture rain and send moist air up into the battery, ballast, air intake area. Given my failed OEM ballast had clear signs of water damage, I decided to skip the inlet.

As to the rest, I agree. Removing that inlet will decrease cool air getting to engine. But with my driving style (engine most likely never revved past 3k, definitely never past 4k), I figured the hotter air wouldn't hurt anything.

 

Not critical. As I discovered doing work after a shop rebuilt my front end (Hit a deer), this piece was left out. Literally went without one for years.