Ars doesn't do AoE very well, not like this potential scatter pattern. It's easier to do a many to one attack (multiple missiles, one target) than to do this many to many attack.
First issue is the base guideline used. You're using the guideline used for Wielding the Invisible Sling, if you want it to be finesse based, you need to use the guideline in Houses of Hermes: Societates on page 38. You're moving them unnaturally for a fraction of second and then allowing natural motion to take hold. So, to take one sling stone and fracture it, and make 10, sure, I'd allow that. I'd had an earlier post that I deleted that suggested some things about reducing the damage per bullet, but I think, all things equal, it's not overpowering to allow +5 damage.
So Wielding the Invisible Sling with a +1 Perdo Requisite and a +2 Group means it's a 25th level spell and you do 10 +5 attacks (I'm keeping the perdo requisite, because he's taking one sling stone and turning into 10, without any increases in mass, while a similar T:Group spell would have to have all 10 stones). How does one target these 10 projectiles becomes the next question. Many to one, I'd have it that finesse total - defense total >=10 is all hitting, and for each increment lower, there's a missed projectile. Many to many, I'd have to say that they would be rolled individually, and like multiple casting, each would get a targeting penalty similar to multiple casting. Note, I'm not applying a targeting penalty per projectile when you target only a single entity. And if you targeted two entities, I'd only apply a -1 to each finesse roll, two entities would be -2, and so on. This isn't, exactly an Area of Effect attack, it's a scatter attack, and there are a limited number of projectiles involved.
But, +5 damage is not a lot of damage, on average it's 11 damage, and the opponent gets their soak bonus AND a die roll, even after you hit. Having a soak of 5 is not hard, so on average, these attacks will do very little actual damage against even lightly armored opponents.
Or going the other way... If you want to do more damage, use a higher level damage guideline for the finesse based spells. If you want to increase your ability to hit, increase finesse and invest in spell mastery, precise casting.