I can only reply with my experience, which is this. I have an ERX SFP running on a Comcast 100/20 connection. With QoS off, I get 120/20 pretty regularly.
With smart queue on and the bandwidth set to 110 down and 21 up, I get about 80 down and 17 up.
So, I've disabled the QoS. I'd like to know why I can't get the full line rate - it should be within the router's abilities.