Hello again, I tried a few more things like switching the USB devices in the USB ports to see if it is hardware related, changed the cable but the problem stays.
The first device ALWAYS get like 1-2MByte/s, the second ~5MByte/s, I wonder what would happen if I had a third to plug in... I even installed Linux (Ubuntu 5.10 Server), to check if it is driver related and now I get around ~11MByte/s which is the max. supposed throughput. So in my opinion, it is either an USB or AXE driver problem within OpenBSD 3.8 and snapshot. So again, anyone got an idea on how to solve this problem? Any help would really be appreciated. Michael Lechtermann