Are you sure that those VR benchmarks are Crossfire friendly?
Your issue looks like well known negative scaling effect.BF1 suffers from it for many months. You need to disable Crossfire for such applications(in game/applications profiles)
2.Update chipset drivers
3.Delete C:\AMD folder
5.Use Heaven 4.0 (DX 11 Full screen High settings AA4x) to test your GPUs scaling and stability. 30 min is quite enough(just leave it in fly mode)
I've updated the BIOS, reinstalled all the drivers using DDU in safe mode. I've reinstalled the drivers and ran Heaven Benchmark.
As you can see, it's scaling almost perfectly, (About 80%):
However, that only makes it harder to explain why the SteamVR test did not scale with the RX 480s. I used to own a Radeon 7990, and it scaled fine in crossfire. I know for a fact that it supports it, so why is there an issue?
I've also found that some games haven't been scaling well, (Watch Dogs 2, GTA 5), while some, such as DX: Mankind Divided have been, but only at a 20-30% increase in framerate, compared to up to 90% with my 7990.
1. 7990 based on GPU launched in 2011.It has quite polished drivers.
2. RX 480 is 2016 product and its drivers are far from perfect.
3.I had noticed that AMD Crimson driver doesn't like CPU limited condition. Especially on crossfire systems.
4.You can use VSR resolution and very high quality settings to avoid CPU overhead situations.
5.Watch Dogs 2 is unstable for now. This game needs patches and new drivers.
6. GTA V is old and CPU bound game. You need VSR resolution (4K) to reach good CF scaling
7. SteamVR... I don`t know. Maybe it needs new drivers or new SDK, more optimized for new hardware.