Not sure whether this is supported in MT, I guess not yet, I would expect to see an 'XR' switch to enable/disable.
thanks for the links, i have seen d-link has boxes (e.g. DWL-2100AP) claiming to use XR. as the white paper states, it works by improving sensibility down to -105 dB and lowering bandwith below 1mbps so it seems to me a technique mostly for the low-cost 802.11g market.
if it is an atheros chipset feature, it should probably not too difficult to make an XR option-switch. nice to have, but not very important.