Configuration for speed up ocl ?

[Mem5]

Hi,

I tested oclhashcat on my new HD 5870 (1600cores@850Mhz)
I read a speed estimation of 5018M for single MD5.
But I only have :

Speed........: 1712.6M c/s Real, 1399.0M c/s GPU

1) How can I speed up ? Which values for "--gpu-loops" and "-n" ?
2) Do you advice to overclock it ?

Thank you.