I am a former racer (cat 2). Every ounce does not count in amateur bike races, because such differences are dwarfed by differences in fitness. Significant figures. Aero drag matters a lot, and most of that is from the rider.

Races sometimes are very close, but they are that close only because the pack or a breakaway finish together. When the sprint happens, there are only a few seconds for riders to pull away from each other. In these cases, minor differences in efficiency still don't matter because it is over such a short time frame.