Are there downsides in using cat5 / cat5e cable for USB 3.0 connection?
No. USB 3.0 uses 9 cables internally. 

If you use the scheme given in the question, you will effectively end up with USB2 which has its own cableset within USB3. 

See USB Cable Diagram. 

It might be possible to combine the two ground wires and use 2 STP pairs for the two Superspeed lines, but I found no reference of this working.