I’ve looked and I haven’t found any Unreal Gear VR demo’s that are performant. They run at 60FPS for a while but if you look around, or move your head too fast it often drops to 30FPS or less. I wanted to send a friend what kind of quality drop we’d have to do if we wanted to try to support rendering realtime on the gear. In my tests which were back in the 4.8 days (I think) on the S6 I couldn’t really get anything other than a static scene, with very few objects.
Are there any well known Unreal Gear VR demo’s?
Hey I don’t know about any demos but there are some released projects that perform fine 
Here are the UE4 gearvr releases I know about:
We Come In Peace…: WE COME IN PEACE... on Gear VR | Oculus
Annie Amber: Annie Amber on Gear VR | Oculus
Escape Velocity: Escape Velocity on Gear VR | Oculus
Gunjack: https://www2.oculus.com/experiences/gear-vr/969815736436279/
Star Phoenix: Star Phoenix on Gear VR | Oculus
Disclaimer I made We Come In Peace… hence the reason I have it at the top of the list 
The best way to get good performance is to read all the mobile optimization tips and at least for my project since 4.11 when the engine added mobile support for instanced static meshes to help with the draw call limit
thanks for that! great list.