You should have no trouble at all running the 575V motor on 380V. The issue that you need to watch out for is that you do not exceed the FLA (current) as shown on the motor name plate. You certainly will not get 15kW - but to run this uncoupled and test it - not an issue. Motor repair shops do this all the time on much more expensive motors (for example 13.8kV motors run tested at 4160V and the reason is that 13.8kV test gear and equipment is very expensive). The motor will still run at close to the name plate speed (3 phase motor speed is determined by the # of poles, the line frequency and the "slip" of the transformer action - voltage and current are a lesser influence).
I have run 575V motors on 460V even with substantial load (talking 100HP motor loaded to 60HP) with no problem. Whether this will work for your RPC application depends on how much load you will have on the RPC. If you are only pulling 5HP, then you should be fine, and can easily confirm this with a current clamp.
Let us know how you make out. David