Extra pierce for the shuriken attacks are quite useful for early-game, especially in combination with Seeking Shuriken. It has little improvement for Upper Path 2 or Upper Path 3, as it only affects the shuriken attack, however.
Using a Ninja Monkey with Sharp Shurikens can be used for early game because of its high pierce, fast attack speed, low cost, and camo popping power.
Crosspathing this upgrade with Upper Path 3 is near worthless, as the Sharp Shurikens upgrade only affects the shuriken itself.
As for crosspathing with Shinobi Tactics, it's generally unnecessary to buy this upgrade, as the extended buff range from Ninja Discipline is sufficient. But adding in Sharp Shurikens can be helpful if a majority of the DPS in the game comes from spamming Shinobi Tactics.
Sharp Shurikens did not receive any balance changes since it was first released in BTD6. However, it can no longer have both Distraction and Seeking Shuriken at the same time, as those upgrades are mutually exclusive as crosspaths for this upgrade.
BTD6 upgrade icon in upgrades menu
Crosspathing in BTD6 (Top: Path 2, Bottom: Path 3)
Sharp Shurikens BTD6 upgrade icon
Instamonkey icon for Sharp Shurikens
Light-grey shuriken, from Ninjas with Sharp Shurikens or higher
Shurikens thrown by a Ninja Monkey with Sharp Shurikens or higher will throw light-grey titanium shurikens instead of dark-grey iron shurikens.