That makes sense, I only get ~420kh/s from the 7870 and it certainly likes single thread/high intensities.
I have some r9 280x cards with the newest beta driver working just fine, though these seem to run better dual thread/low TC and intensity.
My 7870 machine is still on 13.4 because I'm lazy, though. Might be worth a try
Seems like you should be able to get at least 350-375 from that card...
You used the setx command right? I think that will severely limit the max TC, if you forget it or don't have it set right.