Ashes of the Singularity is a game AMD had a big hand in making. There are driver level optimizations the studio paid for. Furthermore the game leverages D3D12's Async Compute which Pascal Architecture can't handle as well as GCN.
That being said in non-AMD sponsored games they two set ups in question (RX480 CFire vs GTX1080) do manage to stay within step of one another [1].
The real issue becomes overclocking and crossfire.
The GTX1080 has a much higher overclock threshold then RX480 (currently this may change as board partners make custome PCB's). So if you start fiddling with power levels this comparison breaks down.
Also crossfire/SLI is to a degree Russian Roulette. Studio's have to opt-in to supporting it, and sometimes don't. Leaving you only with a single card. Also very weird bugs can sometimes crop up. Some people have no issue with this, some have huge issues. YMMV.
The main thing I'm trying to express is when looking into hardware reviews CPU/GPU's more information from more sources is always better. And all sources/chips aren't equal.