I just added a DWA 652 to my Toshiba laptop - used the latest drivers and could not get above 54 for speed. I had to have the router at mixed g/n because the DWA 652 would not connect at n only.
All my other computers have n only adapters or are new MacBook Pros.
I then used the Atheros v7.6.1.221 drivers for the 652
http://www.laptopvideo2go.com/forum/index.php?showtopic=14858&pid=75167&mode=threaded&start=
Still just 54 for speed.
I then installed the 1.22b05 firmware for the DIR 655 and presto - 300 speed.
So I have set the DIR 655 to n only, WPA2 encryption 20/40mhz.
Life is good.