P3 and Motion Hole are not the same thing.  (Motion Holes are typically not suggested for asymmetricals)

Second thing, I will always recommend a bowler try a ball before adding any balance holes.  Even in today's hi-tech environment, we can't predict exactly what a ball will do once drilled. 

If you like using the Dual Angle method, then go through the steps and layout your ball to meet your needs.  Then, throw the ball to determine if you need a balance hole or not.  (I would suggest more than 1 day of bowling, sometimes we have varying releases so trying on different days will confirm if what you saw the first time is correct.)

If you decide you want to tweak the reaction with a balance hole, then go about that process.

Just make sure you set the ball up properly to ensure you can maximize the effects of a balance hole.  I have even had balls be illegal (static weight) before adding a balance hole.  If it is, then don't use the ball during competition, practice only.  If you like the reaction, then a P2 hole will neither add or subtract significant changes in reaction.  Adding a P2 in this instance will bring the ball back to specs.

One thing I have experienced in the past is my pro shop has not drilled asymmetricals properly for me.  For those who use the Dual Angle method, the Pin-PAP distance guidelines are different for asymmetricals than they are for symmetricals.  How much different is up for debate in my head.

With that being said, I do believe the effect of a balance hole on a drilled ball can be significant.  In fact, the originally drilled ball can be drastically different than what you want, but with the addition of a balance hole (and surface changes) you can come up with something better than expected. 

Here is a brief description of the Balance Hole system as described by Mo Pinel:

http://wiki.bowlingchat.net/wiki/images/4/4a/Gradient_Line_Hand_Out.pdf

Remember, when you add the balance hole the ball will be asymmetrical, all balls are after drilling.  The difference is that the psa for a ball with an asymmetric core will be located very near the mass bias mark.  For a ball with a symmetric core the psa will be located near the thumb hole, best estimate without a Determinator is 1/2" to the negative side of the grip centerline and 6 3/4" from the pin.  After you have the psa location determined the gradient line can be used, but you won't be able to use the P4 position because it will be too close to the thumb hole.  And a P3 hole may require pitching the balance hole away from the thumb hole to avoid intersection.   --  JohnP