Hi -
Fragmentation is broken in current Madwifi. Every packet bigger than the fragmentation value will not be send. This is bad, because fragmentation is pretty important for smooth data transfers.
Another notorious problem is IBSS cell-splitting, you can fix that by using
iwconfig athX ap 02:CA:FF:EE:BA:BE (or whatever cell-id you wish)
Also it is recommended to select the ministrel rate adaption mechanism when loading the driver.
Try a recent Madwifi version from OpenWRT Kamikaze (i.e. get the OpenWRT patches and apply them to the Madwifi version used in OpenWRT).
cu elektra