So I'm changing some stuff in EB, but maybe I should think on it some more...

[Titus Marcellus Scato]

My opinion on slingers is that EB has their stats and AP ability just right - but the units contain too many men.

Unlike archers, who can stand in rows 10 deep and everyone can still fire by firing up into the air, dropping their arrows down on the enemy, slingers should only be 2 rows deep, but in a very wide formation. Make a slinger formation too deep, and the men in the rear will start hitting their comrades in front of them as the enemy close the range, since sling bullets travel on a much flatter trajectory than arrows!

Therefore, I think slinger units should be only half the size of archer units. Only 30-40 men on Large unit size, 60-80 men on Huge unit size. Standard formation, only 2 ranks deep. With their upkeep remaining the same, since training a slinger takes longer than training an archer, the sling is harder for a novice to use accurately. Slingers were very skilled people.

So you have the choice of buying slingers for the armour piercing ability, or archers for the greater firepower provided by having twice as many men in the unit.

Source: http://www.lloydianaspects.co.uk/weapons/sling.html