The lock in place targeting priority is mostly quality of life, as there exists tricks to keep the Dartling Gun to keep firing in the same direction while still controlling the mouse in all versions. On PC releases of the multiplatform version of Bloons TD 5, the borders of the screen do not update the aiming direction, while on Bloons TD 5 Deluxe and the Flash version of Bloons TD 5, the player can right click on the screen to open up the flash settings to stop mouse updates.
Due to how expensive the Dartling Gun's upgrades are, the discount can be worthwhile for reaching the tier 3 upgrades faster.
On the Flash version and Bloons TD 5 Deluxe, with Faster Barrel Spin but without also including Jungle Drums, the speed bonus is not noticable due to the speed cap. Avoid buying Faster Barrel Spin with this specialty building enabled, or purchase Jungle Drums on a nearby village to get value off of the speed.