Make impellor changes gradually.
Have it rebuilt with a new wear ring to tighten up clearances and all edges cleaned up to make sure you initially have a properly working pump to set a base line for your upgrades.
Then you can have the impellor gradually cut down till it puts you in your RPM range.
B might be too much cut off considering you haven't even tuned the motor and and you say the pump is slipping.
-beerjet-