How these laser show systems work
It's not a motor, it's a galvanometer: a system which produces a deflection proportional to current. It's very different from a stepper motor as there are no discrete 'steps'.

A webpage for an example professional galvo unit:
http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=3770

That does the courtesy of explaining both how it works


  The angular orientation (position) of the mirror is optically encoded using an array of photocells and a light source, both of which are integrated into the interior of the galvanometer housing. Each mirror orientation corresponds to a unique ratio of signals from the photodiodes, which allows for the closed-loop operation of the galvo mirror system


...


  The Proportional Derivative (PD) servo driver circuit interprets the signals from the optical position detecting system inside the motor and then produces the drive voltage required to rotate the mirror to the desired position.


and also the angular precision of 0.0008° (15 µrad) if you use their special clean power supply.

The light show unit doesn't specify an angular resolution, only a scanner speed in "points per second".