Writing to block device is *slower* than writing to the filesystem!?

Hi all,
we have a new machine with 3ware 9650SE controllers and I am testing
hardware RAID and linux software MD raid performances
For now I am on hardware RAID. I have setup a raid-0 with 14 drives.

If I create an xfs filesystem on it (whole device, no partitioning,
aligned stripes during mkfs, etc) then I write to a file with dd (or
with bonnie++) like this:
sync ; echo 3 /proc/sys/vm/drop_caches ; dd if=/dev/zero
of=/mnt/tmp/ddtry bs=1M count=6000 conv=fsync ; time sync
about 540MB/sec come out (last sync takes 0 seconds). This is similar to
3ware-declared performances of 561MB/sec
http://www.3ware.com/KB/Article.aspx?id=15300

however, if instead I write directly to the block device like this
sync ; echo 3 /proc/sys/vm/drop_caches ; dd if=/dev/zero of=/dev/sdc
bs=1M count=6000 conv=fsync ; time sync
performance is 260MB/sec!?!? (last sync takes 0 seconds)

I tried many times and this is the absolute fastest I could obtain. I
tweaked the bs, the count, I removed the conv=fsync... i ensured 3ware
caches are ON on the block device, I set anticipatory scheduler... No
way. I am positive that creating the xfs filesystem and writing on it is
definitely faster than writing to the block device directly.

How could that be!? Anyone knows what's happening?

Please note that the machine is absolutely clean and there is no other
workload. I am running kernel 2.6.31 (ubuntu 9.10 alpha live).

Thank you