Actually, fighting with knives *is* very different from fighting with swords.
With swords, you may slash or pierce, and parrying is an every-second occurance.
When fighting with knives, slashing is rrare except to sever digits or to hamstring, and parrying is virtually impossible do to the speed and tiney size of the blade. Knives are also more acurate because of decreased momentum.
Personally, I'd make the sneak attack damage d4, then put a few limiting factors on it, making it more like the Ambush ability from the Outlaw of the Crimson Road (song and silence).
Ditch stunning slash, it does not have anything to do with knife fighting. Instead, try something like the ability to target specific areas of the body, such as the wrist vein, the finger, or the eye, usable 1/day/level.
-jeph