What is your settings with the 15kh/s hash? What did you get with BTM + ETH?
90 PL is a good setting, sweetest spot is like arround 70/80% PL but 90% is pretty fine for the card.
Depends of your fan, temp, airflow etc.
Dual mining is trading 6-7k on BTM for 40mh on Ethash basically, but with a bit higher power consumption (due to high memory clock).
So it depends on what you need ^^
I finally was able to test dual mining but I'm getting way different results. I get 6.5k on BTM and 52mh on Ethash at 90% PL.
If I understand your post correctly, you're getting 8-9k on BTM and 40mh on Ethash? What's your overclock settings?
I have no issues with solo BTM tho, getting 16.7k at 85% PL. Seems 85% is the sweet spot for my card.