As it turns out the times reported by GoPro are pretty bang on under ideal conditions. Using alternative batteries does increase the output times by a little.
Please note there are a variety of conditions that can affect run times, such as:
Cold weather, which will reduce battery-life.
Enabling Wi-Fi, which will reduce battery-life
The above times are based on doing a continuous record. Stopping/starting recording, or powering camera on/off repeatedly will reduce the total record time.
I’ve switched off all three LEDs and the sound on my Black edition to save a little battery power.