Unfortunately many pump manufacturers do not publish this information along with their pump curve. With these types of pumps you can reduce the vane diameters, but the shrouds should remain untouched. Structural strength is a consideration when deciding how much to reduce the vane diameter in double ended pumps because you could leave to much unsupported shroud. Some manufacturers recommend an oblique cut that will improve the vane exit flow and add some strength to the shrouds. Machining a radius where the trimmed vane meets the shroud is another good idea to add strength to the assembly.
Square corners are never a good idea.
Under filing the exit vanes is usually a good idea. One look at the above diagram will make this obvious. The exit angle of the fluid will change resulting in a higher head at design flow, but no change in shut off head. Because of reductions in the wake of the fluid exiting the vanes. The efficiency of the pump should improve slightly.