Ok, so it looks like everyone else is getting approximately the same hashrate difference for R9 390 vs. RX 480. I get 260 H/s and 190 H/s respectively with mine (both 8 GB). So my question is, is it expected that the 480 hashrate is only 73% of the 390? Is it because of the lower power consumption on the 480? With every other miner/currency so far, my 480s hash at about the same rate as my 390s. Is this something new with v8.0?
No it is not. It is because 290/ 390(x) are high end cards vs 470 low-mid/ 480 mid level, amd has not released that high performance gpu yet.
470/ 480 owners were lucky to enjoy hig end hashing rates at one algo with some bios modding. Today ancient 290 hashes more than 470/ 480 with the same or better hash/ watt ratio, better get used to it.
ps. what where those every other currencies where 480 matched 390?
While R9 serie has better hash, I don't quite agree about the hash/watt ratio. My RX 470 has better hash ratio than my 290x and the R9 is undervolted, not the the RX.
Look at your 290x bios with hawaiibiosreader, pay some attention to those powerplay tables. Try those core speeds listed in dpm 2-7 + every combination listed in memory dpm 2-7.
Add afterburner -100mV or trixx -200mV and try what works best for you.
) watts is hardly accurate either. i can see my 390s being closer to those temps than my 380, no way it's under 40 watts, but there's no way its anywhere near 220 either. 
