Can't speak to the settings of the app.. but above is accurate.
Voltage directly correlates with battery use. Higher voltage = slightly faster battery drain... Some people actually underclock/undervolt down to 800mhz specifically to get amazing battery life on the X.
And the heat issue isn't just running hot constantly... The chip has tons of solder points on the board.. which is where 2 different metals are connected via a third (solder) to make the connection. Since the different metals expand and contract at different temperatures.. lots of repetitive hot-cold-hot-cold-hot-cold... (I could keep going..
will eventually cause weakening and/or poorer connections. ALL circuitry has a lifespan due to this, but this is one reason that you repeatedly hear that overclocking can shorten the lifespan of chips/devices/etc.
Unless you overclock to extremes... the risk is usually minimal, but as with desktop (and any chip for that matter)... the risk IS there. This is also why some devices "lose" overclock-ability over time. If pushed too hard, some devices can't overclock to what they could previously... what "seems" to be stable may not last.
As with desktop overclocking, I believe the recommended method is to find the breaking point then back it off a notch, then back it of another notch.
You may not be getting the very most out of your device with that method, but it will hopefully help with the longevity of the device and it's stability...
HTH!