I'd estimate you'd get ~465MH/s @ 1250 core, based on me getting ~375MH/s at stock (GHz edition). Note the 7870XT has 1536 cores compared to 1280 in the 7870. I've never tried mining in windows, so I can't help much - sorry.

I'd concentrate on your choice of mining kernel (I find diablo and poclbm to be the best) and stick with the 13.1 cats. AFAIK the SDK is built into the latest drivers so you shouldnt need to install it separately unless you want to use a different version.

Maybe you should do a clean install of the AMD drivers (use driver sweeper in safe mode after uninstalling for example), don't reinstall the APP SDK and test the parameters with diablo and pocblem miners. You could also try disabling the windows7 aero stuff.