I think you are right, eighter HT40+ or -, and it seems I was wrong about the option 'hwmode' 'na' should be option 'hwmode' '11na'
Another thing is that it seems that a lot of the options is for madwifi not ath9k.
something like this should be right:
config 'wifi-device' 'radio0'
option 'type' 'mac80211'
option 'macaddr' '00:23:6c:bf:a0:01'
list 'ht_capab' 'HT40+'
list 'ht_capab' 'SHORT-GI-40'
list 'ht_capab' 'DSSS_CCK-40'
option 'country' 'US'
option 'hwmode' '11na'
option 'channel' '44'
config 'wifi-iface'
option 'device' 'radio0'
option 'network' 'lan'
option 'mode' 'ap'
option 'ssid' 'XXXX'
option 'key' 'XXXX'
option 'encryption' 'psk2'
option 'frag' ''
option 'rts' ''