Yes, I have about 6 towers up using a 532 in this type of config. My largest one has 40 clients on it with an Nstreme backhaul and I'm running some p2p queue rules, and CPU hovers around 20pct. So far it works like a champ. Some of my towers have been going nicely for about a year and a half now.
I personally think your going to run into a bottleneck with just 802.11 and the wifi card before you affect the cpu load of the PPPoE subsystem.