sorry, i dont have any ideas about why you are unable to set the txpower, when i get home i can try the command you specified on my ambicom.
but i fail to see how adjusting your transmit power would have any effect on kismet's sensitivity in detecting APs. kismet does passive detection, as far as i understand it, your card need not even send out any packets when your are running kismet in the detection process.
i suppose changing the txpower on a card that supports it could also change the recieving sensitivity, but it would seem to me that this would all depend on the specifics of the card firmware/chipset.