If that leak is true then that doesn't look too bad.
My R9 290 overclocked to 1115/1500 got 10982 last time I ran Firestrike.
They're saying the RX480 system got 11983. Now assuming my system is not massively discrepant from what they used to test and my R9 290 overclocked was similar to an R9 290X/390X or perhaps a little faster, then that's a bit faster again, just shy of 10%, and that's before any overclocking headroom is considered, which if the 1500MHz+ rumours are true, could put this score up by around 20%.
Obviously no idea what the test spec is...but still. I'd be happy with that if it was around £200, ran cooler/quieter/lower power and overclocked as well