I can't open your capx, for it uses a third party plugin, but I understand what you mean.
There are multiple ways to do this, but here's an easy one (It's not the best solution, but it will work).
Create a global variable called TopSpeed
Set it to the original bullet behaviour.
System compare two values : bullet.speed < TopSpeed
You should do that for each of your objects having bullet behaviour
Then add the event to change the variable TopSpeed to a higher number when collecting a boost.