PDVD 8 Patch 1622 will break DXVA HW assist for MP4 / AVC discs exactly like 3730d did for 7.3.
BEFORE installing the 1622 patch, go to C:\\Program Files\\CyberLink\\PowerDVD8\\VideoFilter and copy CL264dec.ax to a safe location. After installing the patch, go to C:\\Program Files\\CyberLink\\PowerDVD8\\VideoFilter, rename CL264dec.ax to CL264dec_O.ax and copy your saved CL264dec.ax to the subdirectory. You will be good to go.
This works for me, but I take no responsibility for your environment.