Currently this enhancement is nigh-worthless, especially for all the costs associated with expenditures of AP.
I propose that the Way of the Assassin enhancement work more like certain Assassin prestige class abilities, without adopting the entirety of the class features. While activated, the Way of the Assassin boost could add the character's intelligence modifier to base damage, paralyze the opponent on a critical hit, and on a confirmed critical on a natural 20 cause outright death (just like a vorpal, but with any weapon). These things would take place only during the time the boost is activated.
Currently there are very few benefits to this enhancement line, and I'd like to see it made more viable.