1 and 4 runners are aimed better from the start. Their final angle is more towards the center of the turbine inlet. The goilet directs the flow well. Removing iron above those two helps the goilet do its job.

The extra material removed is more to assist 2 and 3; which are aimed across the plenum at that lower side. Now their flow is directed upwards into the turbine with a less severe angle.

While I cannot undo the shorter length of #2 and #3 runners, I can give them an easier path. That will help in a small way to balance the flow. It may not be much; but every little bit helps.