My reaper settings for the 7870 LE (Tahiti XT) are:

GPU 855
Mem: 1500

worksize 256
vectors 1
aggression 20
threads_per_gpu 1
sharethreads 24
lookup_gap 2
gpu_thread_concurrency 24000

Sometimes I use 16384 instead of 24000, it usually depends on the pool. I get an easy 400-420 kh/s with these settings.