I ran my tests with our dd and also with /usr/pkg/gnu/bin/dd, supposedly the same or similar enough to the one in Centos; there was no significant difference between the two. The fastest figure came on the system disk when it was attached to an IDE controller with ICH6 chipset. about 180MB/sec. All other combinations return between 110 and 160 MB/sec. Tried with/without host os cache, also there is a setting that the disk is solid state. No apparent difference.
My host system is build 17120, so that may explain something. Not the difference in figures though, comparing to Centos.
Chavdar