The 7950 will not run vr as a single card. So in cf it works & gives a steamvr rating of 3.5 eith the odd 6.0 result.
While games may not directly have support tge vr runtime does. I don't know about nvidia's sli.
But without 2 7950 in cf my pimax would have been a paper weight without buying a new gpu.
So i submit your wrong about some of the multi gpu support. We had a user bound & determined to have 1 7950 work. I disabled crossfire got an very low result in steamvr & every game crashed until i reenabled crossfire.
As for the r9 390 8g get mid high results on steamvr perf test & it challenges nvidia 980.
So i know i should be fine. Lol
And unlike nvidia's nonsense with cards like the 1060 i can scale another card in. But i am sure the 1060 6gig card should do fine as well with the 8k.. Possibly even the 3gig model may work better thsn expected.