With bufsize = 2^27 = 134217728, -I 12 and -w 32: 40Kh/s !!! This is surprisingly good.
However, I cannot increase the intensity further because then I get 0Kh/s and and a process so that I have to reboot.
Edit: BTW, I am hashing at http://prometheus.phoenixcoin.org:10554/static/ in the moment