Other thread, same question:
Of course it will affect filtration. Any method of flow reduction other than the one built into the aquaclear will reduce filtration.
The pantyhose trick is usually used to protect small critters from getting sucked into the filter, I had not heard it was useful for reducing flow rate.
You could either block flow into the filter somehow, that could prematurely wear on your motor though; or you could order a spare impeller and cut the fins a bit short, reducing flow without hurting the motor.
Why do you need lower flow?