12 MH/s is already very fast for the old 7870 card. You can try --cl-global-work 16384 --cl-local-work 64, maybe 2-5% increase.
Also try bumping your memory and core clocks up I have a 270x which is basically a 7870 that I OC to 1200/1500 that averages about 16.5 MH/s using the global work flag