Reviewing the description of your installation, the biggest problem involves the impeller dimensions.  Their location is as good as practical, but you have impellers with many (8) blades that are very narrow (1/16 of the diameter).  The narrow blades are especially bad with a 16,000 cp viscosity.  As viscosity increases, the flow pattern, even from an axial-flow, pitched-blade turbine, becomes radial flow.  With narrow blades the effective capacity of the impeller to move the fluid is dictated by the vertical height of the blades.
 
Your application could be improved considerably if the current impellers were replaced by 45 degree pitched-blade turbines (PBTs) with four (4) blades and blade widths about 1/5 of the impeller diameter.  By my estimates, 800 mm diameter impellers with four 160 mm wide blades may require about 10 kW.  Slightly smaller impellers, 750 mm diameter with four 150 mm wide blades may require 8 kW.  Those power estimates may be a bit high, since the interaction between the mixers in a rectangular basin are difficult to predict from standard results for cylindrical vessels.  I don't want you to overload your motor.  My estimate for your 800 mm impellers with eight 50 mm wide blades is 6.5 kW.

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