I found dd to be a very bad/misleading tool for this case. Problems are caches in different layers of the system, filesystem behaviour, sector sizing of drives and arrays, kernel configurations, input data loading, real world scenarios and driver implementation. I had same issues on centos. Not perfect but a lot better for my purpose is bonnie++. Even with bonnie++ i would not dare to say that same tests on same hardware with centos and openbsd will show the real differences in performance.
Maybe that might help to get more comparable results
