The question of maximum viscosity has different answers depending on the limiting factor.  The 1 hp motor will not overload, even up to 50,000 cp.  However, the mixing will not be effective above about 1,000 cp.  In effect, the motor is larger than needed to operate the impeller and the impeller is smaller than needed to effectively mix the tank contents.
 
Increasing the impeller diameter to 18 inches from 13 inches will not overload the motor and also increase the effective mixing limit to about 7,500 cp.  Even a 20-inch impeller may work, depending on the specific power number for the hydrofoil impeller.  A quick check of the shaft design does not show a critical speed problem, even with the larger and heavier impellers.