Yes, the radius is overkill. Most things that are manufactured are designed to withstand much more abuse/force than required...that's why you've never broken one at the radius. In manufacturing a radius is almost always called for except in unique situations. I would be interested to see if the factory pinion has a radius where the "undercut"" meets the input shaft?? I would bet there is, otherwise they would have simply put a sharp corner there without the undercut. The undercut w/radius doesn't weaken the input shaft or the rest of the pinion but a sharp corner would absolutely make it weaker.
