Cool. I didn't know that (BFGMiner documentation seems a little incomplete or i have overlooked that). Is it also possible to set the clock speed at runtime? Haven't found an option for that in the Menage Devices part.
Ok now I ran into another problem. I've been running the U2s at 1.4 GH/s for a while because the were getting so hot. Because people now stated numerous times that 1.6GH/s shouldn't be a problem even when the get quite hot I wanted to run them at 1.6 GH/s. So I started them with the higher clockspeed only to find that one of the two would become etremel slow after a while (0.5 GH/s). when back at 1.4 GH/s setting the both run fine again. Any Idea what could be causing that?