After working out some quirks I finally got around to testing the RX480 full out on LN2. It’s not incredible but not all bad. This specific card was not great to begin with so I think we can see up to 1800MHz on a better sample.
Highest fully validated I did at 1650/2075 and with Tesselation disabled in the driver, see my submission here.
For LN2 overclocking you need a BIOS which has the cold slow issue fixed (at below 0*C it will lock in low-power state), which will be available soon. I used the eVc for voltage control, more info at HWBot forums.