About energy: it enters the power saving mode when you lock your keyboard/phone, so no impact there.
Energy impact when active depends on your usage pattern of other applications: shortcutd is not heavy on the processor (you can check this with top command) but, as it uses sleep(), it requests a lot of CPU wakeups, which can prevent the processor from entering the idle state. We will work on optifying this, still it shouldn't have any serious impact on your battery life at the moment. Any feedback on this would be appreciated

Disabling sensor with long touch means that it won't bring up dashboard/phone application. The sensor itself is still checked, so you can reactive its functions with long press again. To turn it off permanently, you can use config file (and reboot in this version).

No impact of the application during calls (not really our work, simply the keyboard is locked during phone call with the proximity sensor, so the application enters idle state).