As hashcows has vardiff, I used multiple instances of cgminer - 1 for each card in the system. This allowed me to tune each card individually (7850 & 5870 & 5770) and for each one to have its optimal difficulty.
What thread concurrency (t/c) are you using for your 5870? - i'm running 6144 atm but i'm conviced it can do better (850 clock - getting about 400)
my 7950 is running at t/c atm 21712 but only getting a very low hashrate