I have always had an idea for a knife effect to be as so:

On backstab:
+30% movement speed for 3 seconds
+50% swing speed/ swing recovery for 3 seconds

However, the base weapon swing speed would be 25% slower and the user would lose 15 health

The idea behind this is that if the spy wanted to perform a chain backstab, he could do it faster using the increased movement speed and swing speed, and afterwards could use the movement speed to make a clean escape.