Looking for a high speed, high powered motor
First off: Why? If this is being used in a vehicle, then you are converting kinetic energy to electric in the alternator, then back to kinetic in a motor.

The point of this is for dynamic response? A mechanical supercharger is still probably better here (but i understand its a lot less fun to build...) Another solution is a normal turbocharger with some mildly clever (fun!) electronics to control a simple anti-lag system. I have some ideas for this...

I have seen electric superchargers before and found them a little ridiculous from an engineering standpoint, due to the immense power consumption normally derived from a 12V source. 5kW does sound reasonable for a small-ish capacity engine with fairly low boost pressure.

The ones I have seen use a small synchronous AC motor, which can easily be driven with a high frequency (2500Hz for 4 poles and 150krpm). Bowever, as you said, this is a bit of a unicorn.

A simpler solution might be to use a starter motor (at least available) and a high-ratio belt drive. The link below use a clever system like this to run a centrifugal blower from the crankshaft

http://www.brennanracing.com/supercharger.html

Also, worth knowing that the centrifugal type compressor will have an exponential response. Double the speed, four times the airflow. Bit of a pain to control. The other option is a screw-type supercharger (look up Opcon Autorotor AB). Bang a starter motor in that and you might be laughing. (Or attach it to the crank like a normal person...)