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 ^^