It has been said the the CS fan is a thrust fan with lower velocity, I don't agree with that. You can calculate the airspeed (m/s) from the thrust (Kg) and the exhaust areal. So if two fan push the same amount it is the exhaust areal that decide the airspeed. Here is a nice link that explain how the physics works. http://www.scribd.com/doc/102868763/Thrust-Calc
The exhaust areal on my mod. Fan with a 40mm motor is 28% less than a 70mm fan mounted with a 28mm motor. This means that the airspeed is 28% higher on my fan if you compare the fans at the same thrust.
I did some calculations on my fan and reach 519Km/t at 3.6 Kg of thrust.
Also I think a cut down CS90 has a better chance of survival at high rpm's. First by cutting it down you will loss some weight giving less load on the blades. Another ting is that you will have a reduced centrifugal force due to the small length of the blades and one more thing, it is stronger build than any other 70mm fan that I seen, exept for alloy and carbon fans.