If you define the speed variable in the instance, then yes, that variable is specific to precisely that one instance. So you would need to loop over all 15 instances of your ball and change the variable for each one of them:
for i in range(15):
get_node("Ball" + str(i)).speed = 400
Alternatively you can store that same variable in a global script (a so-called Singleton) and setup your ball script in a way, that it get's the speed from there:
move_and_collide(direction * Global.Speed * delta)
Then you only have to change the value once (in the global script).