no need to try qubit you was your time, skein is more profitable for amd
just take a look here https://miningpoolhub.com/
than you cheek the average difficulty and you know
but you can setup qubit miner in case, difficulty drops or skein goes to high
so you have an 280x what run at 270?? weird because r9 390 does less than 280mh
and r9 380 does 200mh, i had running 20x 380 for 4000mh but real speed with the rejected are more at 3700
i checked this weekend and the 280x were running at 268MH/s before rejected. i took a screenshot but just managed to truncate it to zero bytes with sftp... i'll take a new screenshot maybe next week.
I'm using this cgminer:
https://github.com/reorder/cgminer_skein