It is not possible for an object to travel faster than the speed of light, what you are talking about is group velocity, which is different from a single particle exceeding the speed of light.
While i am not a physicist, i do understand that as spacial velocity increases, temporal velocity decreases. Put more simply, as you speed up, time slows down. Small increases in speed do not illustrate this but huge increases do. Speed of light travel brings time almost to a standstill from the particle perspective, so anything travelling faster than this would approach zero temporal velocity. Theoretically, a particle travelling at 300 times the speed of light would be travelling backward through time, and would by definition, not exist.
The experiment you describe involves the knock on effect of all of the gas particles amplifying a signal, like as if in a line of dominoes each domino was to begin falling before the previous domino made contact due to something in between being pushed into the next domino. Thus the final domino in the line would fall faster than if it were performed under normal conditions.
I'm starting to confuse myself now so i'm going to stop here!!