So, there you go, you answered the question. The low density of zinc or copper start being a penalty on BC when your bullet gets to the "heavy-enough-for-caliber" limit.
Best to stick with lead core copper jacketed bullets if you are that concerned about BC. Clearly heavy for caliber zinc bullets would be too long causing a number of problems. Twist rate comes to mind along with COAL and bullets taking up too much powder volume. Monolithic copper bullets already have some of these problems along with cost. Copper is more expensive than lead or zinc.
Cast lead bullets with boattails and long ogives are not often seen, cast lead bullets are most often used at lower velocities again where BC is not much thought about. In a lot of applications cast zinc bullets could be substituted for cast lead bullets with a savings if the zinc bullets were available.
In some cartridges we could see a performance improvement where you approach velocities that are too much for lead bullets but the zinc bullets can tolerate. Other cases where we would be inclined to use a gas checked bullet we could get away with a plain base bullet if it was cast from zinc.
Yes, many times a lead bullet will be best and blasphamy but Jacketed bullets would be even better.
Tim