The filler plates make the corn only go through the cylinder once. They can not come back around by falling into the center of the cylinder. I would try to get set of filler plates. Most guys around here left them in all of the time. It may cut back on your capacity in wheat/oats but usually you are not at full capacity in those crops anyway.
You can try the half rounds in the door. The half channels may help but they still have sharp edges and may still damage the corn.
Also do you have the cornhead book for your machine??? Reason I am asking is that some of this is covered in it not the combine book. Gleaner was kind of unusual in that the header books also contained much of the information needed on the combine for using the header.