I once learn from Aerodynamics that if you have a square corner or trailing edge, the airflow will keep going as if the airfoil continues, thus does not create a negative pressure associated with a rounded corner. I can only suspect that a square wingtip will be more aerodynamically clean than a round wingtip. The stock wingtip I think messes up the air going around the airfoil of the outer span, which is the most critical for harrier. Well, it's just speculation. I will see if it makes a difference in pratice.