Bloonsman here, player of BTD5 iOS, blogger on bloons wiki, and expert on every tower, upgrade, bloon, agent, and agent pro. Here is my guide to the Dart Monkey's upgrades.
btd1 and btd2:
Piercing Darts: pops 2 bloons per shot.
Long Range Darts: increases range.
btd3:
Long Range Darts
Extra Long Range Darts: increases range.
Piercing Darts
Razor Darts: pops 3 bloons per shot.
btd4:
Piercing Darts
Long Range Darts
Spike-o-pult: Converts the tower into a slower fire rate but 18-bloon-popping catapult which shoots spiky balls.
Triple Shots: Shoots 3 spiky balls at once.
btd5
Long Range Darts
Enhanced Eyesight: Extra Long Range Darts and Camo Detection combined.
Spike-o-pult
Juggernaut: Shoots bigger spiked balls able to pop any bloon type and damage ceramic bloons extra.
Sharp Shots(Piercing Darts)
Razor Sharp Shots(Razor Darts)
Triple Darts: Tosses 3 darts per shot.
Super Monkey Fan Club: Ability converts tower and 9 more dart monkeys into Super Monkeys for 15 seconds.