Could having too many red blood cells for a 5yo be a good thing?

Doubt it. A healthy blood cell count implys that it is in high enough in quantity to do the work but not so high that it plugs up the system. Blood is a complex blend of cells & liquids that must flow thru our "plumbing". Too many red cels will lead to blockage, like your toilet backing up, and clogs will mean no nutrients get thru.