I'm confused.
The point of the sling was to synergize with ammo, where ammo has not generally been all that good for multiple reasons.
Make ammo better but remove tokens that synergize with it so that it (ammo) is not as useful?
Assuming avoid sling stones that do double damage to things, let's say a sling stone goes from +4 damage to +6 damage. The net effect of this particular token would be +4 damage over what it was already capable of and +6 over a different sling (ignoring, for the moment, Mighty slings that keep getting bigger from all of the STR increases that happen) as the cost of a consumable every time it's used. In a vacuum, that may sound like it's good. Yet, it doesn't get a STR bonus to damage like various other ranged weapons do. Is it going to be any better than a +2 Lt. Crossbow of Frost?
Instead of worrying about ammo suddenly becoming actually worthwhile to use, where I can see it being largely a waste of money most of the time even if everything gets +2 damage, can change the damage wheel on this thing to 3, 4, 5, 6 which is a pretty normal damage wheel for a sling, anyway, including various UR slings over the year.