I've tried only N, only G, N+G, 20hz, 20/40, etc.
I'm from Chile, HW version A1, I even updated the firmware (From 1.0 to 1.03) since it said it improved wifi speeds. It has nothing to do in regards to ISP since I'm testing file transfer on a lan from ethernet at 100 mbps to wireless (Through cable it goes all the way up to 11MBps).
The fastest it went was to 5,5MBps.