Turbo mode should be available on channel 6 only. you can try to set tubo mode manually in CLI(shell) to see if it will do so.
try "ifconfig ath0 list caps" from console, it should output somthing like:
pfsense:~# ifconfig ath0 list caps
ath0=6783edcf <wep,tkip,aes,aes_ccm,ff,turbop,ibss,hostap,ahdemo,txpmgt,shslot,shpreamble,monitor,tkipmic,wpa1,wpa2,burst,wme,bgscan,txfrag></wep,tkip,aes,aes_ccm,ff,turbop,ibss,hostap,ahdemo,txpmgt,shslot,shpreamble,monitor,tkipmic,wpa1,wpa2,burst,wme,bgscan,txfrag>
See: "this post" for a extensive explanation of what is listed here
note the TURBOP (Turbo Protocol capablilty) listed.
If you see on the interface config page i web GUI it should list a bunch of available channels. it should also show which channels (if any) are turbo capable. Normally in G mode it will only be channel 6 and it should be listed as the last choice of the 2.4GHz channels in the channel list, if your card is 802.11a capable it will also list the 802.1a channels and on the bottom it will list the 802.11a turbo capable channels.
Also be aware that the interface will never report 108 as speed, it will allways be 54, with turbo it will connect as 54 turbo. Some manufacturers will list it as 108, but the real speed will never be anything but 54 with or without turbo mode (or 2X mode if you like).
Hope this clears it up for you.