It’s good to hear others are also testing the engine in a CAVE. We also come from OpenSceneGraph and are experimenting with UE4 now 
About the performance problen: I haven’t tried this but it used to be possible to say which graphics adapter UE4 uses, maybe this will help: Multiple Graphics Adapters (related to 4.1 QFE / very low fps rendering) - General Discussion - Unreal Engine Forums
We are actually using the UE4 multiplayer features, it’s hard to get it going but it’s worth it for the actor replication features. If you have animating objects or objects with physics then UE4 can be made to sync them for you every frame (but you need to disable all the throttling mechanisms in UE4 so it replicates all the objects that need replicating each frame.)