As extensive as this description is, it is missing an important piece of information.  The rotational speed of the mixer is missing.  If the speed is 68 rpm, the mixer would effectively load the 40 hp motor and provide good solids suspension.  While "uniformity" is difficult to achieve, especially with an overflow for discharge, the mixer described should give good uniformity of the average particles.
 
    If the speed is less than 68 rpm, then increasing the mixer speed would improve the uniformity of the suspension.
 
    If the particles are larger than 60 microns or more dense than 2.9, then some particles may not may not be sufficiently suspended.
 
    The design as described should provide good suspension and reasonable uniformity.  Greater uniformity would require a larger mixer, but not much improvement would be expected.