Thats pretty low for Sia. I've got a 7970 and it gets 12.5MH/s ETH plus 620MH/s Sia. You probably want to add the -dcri xx argument I have mine set at -dcri 49 and it gives me double the Sia while only lowing ETH by like 0.5 MH. You can tweak the -dcri value in the runtime using the +/- keys. Then hit "S" for stats and you can find your best ratio. Then put that in the start bat.
Try UBQ, MUSIC or EXP you'll get 25 MH/s due to the smaller DAG, I'd also go easy on the SC with these older cards. I underclock mine and run the fan at 100% BTW.
So I redid the thermal paste on my 7970 and temps dropped >20C! Went from 90C at 100% fanspeed to 69C at 67% fanspeed dual mining ETH+SC.
The previous owner slathered on way too much paste and didnt tighten the cooler down enough either. Runs like a dream now.