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
8192 for my 5870 w/1GB
cgminer --scrypt --worksize 256 --lookup-gap 2 -I 15 --thread-concurrency 8192 -o stratum+tcp://stratum01.hashco.ws:8888 -u username.worker -p password
this gets me ~375 kh/sec with stock clock of 850/1200 so I think you're spot on with your config. I had to balance mine with using the machine, so I tend to run a lower intensity on that card as it's my everyday workstation.