I am not suggesting media lose the ability to bring back leaves to the nest. Instead I am suggesting to add in an option for a pheromone group for their ants to drop any leaf carried once the ant has climbed back down a plant. So the ant can climb back up again and harvest some more.
If leafcutter media can cut leaves, drop them immediately after climbing down, climb back up again and repeat. Then they do not need a speed boost to be a viable unit anymore. Instead they become a powerful economic unit when working together with leafcutter minor ants. The media cut size 15 leaf chunks and the minor ants have the numbers to very quickly return the leaves to the nest. Basically making the minor ants carry leaf sizes as though they were media.
For example, if 1 media and 5 minor ants are harvesting, then the ants bring back 5 * 15 = 75 (number of minor ants * media cut size) with a speed loss instead of 15 + 7 * 5 = 50 (media cut size + minor ant cut size * number of minor ants). The advantage of separate cutting and carrying becomes even greater over longer distances, as the number of minor ants per media increases.