Having used mostly PPPoE have not used the rate limiting in the AP/SM much. It would be nice though if the Codel algorithm could be used to improve user experience.
We use fq_codel on all our clients through our Preseem box. Due to CPU constraints I don't think you'd want the radio doing this kind of shaping.
With the SM only handling one queue I dont think it would be too bad. I think the downstream might be handeled in the AP with Canopy gear and it might be handling more then one queue. DD-WRT and Tomato-ARM seem to support Codel so I would not think it would be too demanding if its running on older wifi routers.