in a gestalt game i had a pc running with a rogue//soulknife and at level 3 they were the hardest thing going...
until they met undead where suddenly the halfling was using a dagger to do d3 damage per hit piercing...against zombies/skeletons who have DR5

problem is also once you have iterative attacks you won't be able to utilise psychic strike as often.

assassin after level 16 surely goes into ninja, scout or even arcane trickster (get an item of mage hand, it'll be cheap)
ninja allows you to turn invisible which is always handy, plus adds damage
scout adds speed at level 3, adds initiative and fort saves plus skirmish
arcane trickster increases SA and spell casting