I could not find a performance curve for centrifugal fans when spun too fast. Did find some data that shows the operating curve for a fan (CFM vs RPM) is not quite linear. So I'm going to hold to my claim that if a given fan is spun too fast, the actual output in CFM could go down, or at least level off. I'm going to say that these fans have a CFM vs RPM curve that peaks at some speed beyond its proper design point. I think at some point the faster the fan spins the flow over the fan impellers will separate off the trailing edge and form a recirculation eddy off that trailing edge, and this will spoil the back pressure and CFM will decrease with increasing RPM. That makes sense to me.
FWIW, boat propellers can spin so fast that the pressure along the blade falls below the vapor pressure of the water and water vapor (or dissolved gas) bubbles form -- called cavitation. When these bubbles collapse, they can do so with explosive force, and in some cases can erode the propeller. Even absent actual damage, cavitation greatly reduces the efficiency of the propeller and the thrust it delivers will go way down.