Per device tuning?

[PnkPanther]

Is it possible to set different workload for multiple GPUs?  I'm using hashcat 3.10 in windows with 2 AMD gpus.  One is a older HD 6870 and the other is an older R9 270x.  If I benchmark them separately with -d 1 and later -d 2, speeds are as expected HD 6870 ~35khash wpa and R9 270x 90khash wpa.  However, when hashing using a worlist or mask, the HD 6870 runs at one third of that of the benchmark HD 6870 10khash and R9 270x 90khash. 

Recap:
1.  can we tune each device separately?
2.  Can you speculate about why one card runs slower