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?
290(x) / 390(x) have a much wider bus to make up for the slower speed RAM.
They do NOT hash at the same hash / watt ratio as the RX 470/480 - closer on ZEC than on ETH but the newer cards ARE still more efficient. R9 290s in particular are SERIOUS power hogs even with undervolting and BIOS strap mods via TheStilt.
On the other hand, most folks that have R9 2xx and many that have R9 3xx cards have long since paid them off so anything they're making now is pure profit.
HD 78xx/79xx cards mostly are even bettter off, most of THOSE in mining use were probably paid off by Litecoin (and general Scrypt) mining back in the day, or worst case by Darkcoin (now DASH, X11) mining after that.
Currencies that are NOT memory-hardened are where a RX 480 might match the 290/390 and possibly even the 290x/390x - close on core count, quite a bit higher clock - but there are very few if any profitable GPU-mineable coins that fit that description any more.
If X11, Scrypt, or SHA256 were still GPU mineable the RX480 would probably be a bit faster on those than the 290/390 and possibly the 290x/390x.
Technically, Gridcoin would qualify NOW if you are working the Moo Wrapper project, it runs the distributed.net client which is well known to almost not use memory at all and is TOTALLY compute-bound - but trying to make money via Gridcoin just don't happen, you're doing quite well if you manage to pay the electric from Gridcoin income.
... waiting for version 9 ... 10 ...