Sprint Reactor doesn't have a use.
Blink is better for quickly covering a short distance.
Repulsor is better for quickly covering a long distance.
Fuel Reserve is better for agility.
Repulsor is the main problem, though. Fuel Reserve has its own role. Blink has its own role. Repulsor has the same role as Sprint Reactor: covering a long distance. More speed is not going to fix Sprint Reactor. It needs a longer duration. If Repulsor is going to be the burst option, then Sprint Reactor should be the endurance option. Even with a longer duration on Sprint Reactor, Repulsor and Blink would still have a massive speed advantage and Fuel Reserve would still provide far better agility. This seems like the perfect way to buff Sprint Reactor without breaking the balance between these four gears.