And if you connect a 230V motor to a 400V inverter and configure it correctly, you get a 43% increase of power/RPM 
Please explain as it's a new one on me I spent a few years stator winding when I served my time and the only way to increase the rpm is to up the HZ
Yes the torque will up when you use the motor in a star delta setup as in a S D starter eg low cuernt and torque on startup full power in delta
If a motor is a 1 hp when manufactured as a star only unit then you may see more hp but not any more rpm
Stuart
I'm a motor "rewinder" too. A normal motor, when designed, the core/winding/power relation is calculated around a target frequency, normally 50Hz in Europe. Motors core have a linear voltage frequency relation so, if you apply 25Hz to a 400V/50Hz motor with a standard VFD and measure the voltage at the motor's terminals using a RMS multimeter, you will find 200V, so at 50Hz you will get 400V, the maximum voltage a VFD can produce.
Back to a normal motor's name plate, you will see that a 1,5Kw motor 50/60Hz, it will be 1,5/1,8Kw.
To increase the power/torque over 50Hz, you have to increase the voltage in the same proportion. With a modern VFD, this can be done by connecting a 230/400 D/Y motor to 230, programming this in the motor menu and the 400v VFD will "give" the motor 230V @ 50Hz and raise linearly the voltage to 400V @ around 86,5Hz with the proportional increase of power/torque and speed without straining the motor. After 86,5Hz the speed will still increase but the power/torque will stay stationary.
If you take a normal 0,25Kw 400V 1500RPM motor and rewind it to 100V by reducing the turns and increasing the wire's cross section by a factor of 4, you can run it on a 1Kw 400V VFD to 6000 RPM and it will have 1 Kw power. It's not recommended to go much higher in frequency because of core losses.