Agreeing with Colin regarding environment specific... To answer our questions, disabling the second channel will prevent your radios from performing 802.11n channel bonding. The min and max tx-power sets the constraints for how the radios will dynamically change their power. I highly recommend having no more than 2 or 3 stops (sorry, photography term) in between your min and max. So, if you did an on site rf survey and determined that 12dBm was an appropriate transmit power, I would recommend a min of 9dBm and a max of 15dBm.