P-Torps are going to be very good for Y's as Hrathen said. Double the dice with potential bonus damage. Also it allows them to hit long range with much greater potential of making it count. And since they can take 2 per ship, that only adds to their potential effectiveness.
I would suspect that they'll work best against unshielded ships though. Have an X knock down the target's shields and let the Y-Finish him off.