Sharper Darts is an upgrade for the Monkey Ace that appears in all versions of BTD5, all versions of BTD Battles, all versions of Bloons Monkey City and BTD6.
In BTD5, it is the 2nd upgrade of Path 1 for the Monkey Ace, this upgrade allows each dart to pop 8 bloons (from 5) each, and costs $430 on Easy, $500 on Medium, $540 on Hard and $600 on Impoppable.
In BTD6, it is the 1st upgrade of Path 3. The effect and costs are exactly the same as in BTD5.
As an upgrade on its own, it is not as useful due to the Monkey Ace's decent pierce.
Middle Path Ace is far more focused on its ability, which do not benefit from any crosspaths.
This upgrade provides Top Path Ace with extra pierce which is useful when hordes of bloons arrive and is better than the other crosspath, of which the Exploding Pineapple does almost nothing while Spy Plane provides nothing for the Sky Shredder.