[Sombre]

But you could argue that an artificial boost to strength, such as that from casting strength of giants, is allowing the slingers to overcome the limits of their 'natural' build. And the increase in strength gained through experience alters the damage done by their sling-fire because they are more accurate or have better technique.

Maybe the forumla isn't spot on, but the way it translates into the game works out. If the unit had high strength to begin with, he's using a bigger sling and projectile. If he had his strength boosted by a spell then he's being given abilities beyond what is 'natural' and is more like a slinging machine.

Regardless of this, as a gameplay thing I think I prefer it to vanilla slings, because it allows some nice combos with strength spells, etc and it differentiates slings from shortbows further.