I'm seeing similar issues with the same WD5000KS (B0) drives on a 3ware 9550SXU-8LP. In RAID 5 with 64k stripes, I typically see 7MB (megabytes - not bits) writes and 50MB reads. I'm not a RAID expert, but even with the expectation of "slow" performance with RAID 5, that seems pretty low. Software RAID (md on Linux 2.6) consistently achieved 25-30MB reads and writes with the same drives (I bought the 9550 hoping to speed things up - imagine my surprise). Note that these are long (multi-gigabyte) writes - short writes that use the cache on the card are understandably pretty quick.
I have the same firmware on my KS drives as you've noted below. I'll watch this thread and keep my fingers crossed that WD issues a firmware update to speed things up (or write one of the three drives off as a spare, configure a mirrored set and go get some "known-good" drives for the RAID 5 array).
So it seems there is really a problem with those drives, that 3ware was able to confirm. Here are firmware versions we have ou our drives:
the WD5000KS have firmware 07.02E07
and the WD5000YS have firmware 09.02E09
I also have to add that those drives show no problem in RAID0 and RAID10, they give the same (good) performance than WD3200KS/YS
If anyone heard of a solution for this, I'll be glad to hear... meanwhile, I hope WD gets a firmware fix for this, because we have 16 of those drives and we need RAID5