In BTD5, it is the final upgrade on Path 2, costing $2550 on Easy, $3000 on Medium, $3240 on Hard and $3600 on Impoppable.This upgrade adds the Turbo Charge adds the Turbo Charge Ability, which increases the firing speed of the tower by 5x (hypersonic) for 10 seconds. It is recommended to combine the Glaive Thrower upgrade along with this upgrade; not only will the Boomerang Thrower pop more bloons at once, it will also increase its attack rate even more.
In Bloons TD 6, this upgrade has been buffed significantly. When active, it now increases by an additional 7x, firing at nearly 25 times a second, and also increases damage by +1 for the duration of the ability. The duration is 10 seconds with a cooldown of 45 seconds (or 35 seconds downtime), though there will be a 15 second initial cooldown before initial usage. Turbo Charge costs $3400 on Easy, $4000 on Medium, $4320 on Hard, and $4800 on Impoppable.
This ability has always been a good way to defend against a lot of rushes (such as the notorious RRROD), and has become even more reliable thanks to it now being able to deal with Camo Bloons. However because of this it became very hard to try to kill a player if they brought the Boomerang Thrower in their loadout since they could easily place down a 2/4 Boomer and easily destroy any rushes. Overall this upgrade has been buffed as it's better overall at defending rushes compared to the initial release of Bloons TD Battles Mobile.
Turbo Charge now grants camo detection to the Boomerang Thrower while the ability is activated.