I think you should control the following parameters, ordered at increasing importance/impact.

1. Match the diameter/span ratio of the runner with the specific speed your application requires,

2. Model the trailing edge curvature to extend cavitation margin at part load (I am assuming you do not have guide vanes upstream of the runner.)

3. Improve the seal between the impeller and the casing.

4. Reduce the wet area of the vane without reducing the tangential component of the lift.