Stage I kits are best for vehicles that are stock. - Performance 550 Motors with Internal Fans. Metal Pinion Gears installed already. Easy, fast installation for a 10-15% speed boost.
Stage II kits are best for vehicles that require increased durability. - Performance 550 Motors with Internal Fans, Steel Pinion Gears, Steel First Gears on Ball Bearings, Race Grease, & the newest 7R Gearbox design.
Boomerangs will not tolerate increased voltage due to their electronics. As such there is no benefit to a Stage III or IV kit as they are designed for higher voltages.