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.
Well I'm just using the 7970 as supplementary to my main R9 390 which gets a nice 31MH. Pulling >43MH on two old cards is not bad at all, IMO. As for the SC, yeah dual mining really heats it up. Hitting 91C in a cool room, but I need to replace the thermal paste. Not even going to attempt dual mining on the 390. That thing is already a furnace. I turned it on once and it hit 94C pretty quick.
You think mining one of those coins would be more profitable on the 7970 than ETH+SC? It's gonna take forever to hit the payout in the Sia pool at this rate.