There are known performance issues with athn(4) but it should be able to reliably pass packets.
Do you have two antennas and are they properly connected? 11n mode will not work if only one antenna is connected to the card. You are not hard-coding a channel, so the AP will select one that looks "good enough". Are there any other networks on the same channel which ends up being selected? I would expect 11g mode to work like it did in 6.0.
