5850s can get upto 400MH/s but takes a lot of working and As others advised you would need to adjust and overclock. Set your Engine to 895 and ram frequency to 300 that's if your card will goto it if its working fine try engine to 900. All depends on the card also drivers and also temps and many other factors. You could always try cgminer and program that to overclock in it and see if any speed differences