Just to be sure, are you referring to accuracy or resolution? Looking at your text I reckon the latter.
The maximum allowable/highest resolution of your DEM depends on the Quality Parameter set during construction of the Dense Cloud. When the GSD of inputted images is 5cm the maximum allowable resolution for the DEM is as follows with respect to the quality parameter for the dense cloud step:
Ultra high = 1 * 5.0 = DEM resolution of 5 cm
High = 2 * 5.0 = DEM resolution of 10 cm
Medium = 4 * 5.0 = DEM resolution of 20 cm
Low = 8 * 5.0 = DEM resolution of 40 cm
Lowest = 16 * 5.0 = DEM resolution of 80 cm
So in other words you will only be able to achieve the same DEM resolution as your image/orthomosaic GSD if you set this to Ultra High. Considering you only get 20cm I assume you generated your Dense Cloud at medium quality, correct?