I think it would be a good idea to change the perk "Paralyzing Strike" the following way:

If you hit your enemy IN THE BACK (to attack the enemy in the rear - hope you understand what i mean, it's not my native language) your power attacks have a 25% chance to paralyze the target.

This would combine perfect with the "Quick Reflexes" perk (slow motion if you are blocking during an enemy's power attack).