RAID 5 write performance

Everybody knows that RAID 5 has poor write performance, but how much?
With some spare parts at my disposal, I've done some testing:
Here is a RAID 5 volume, built with 7 SAS drives, each one is 146GB @ 10K rpm.
The controller is an HP Smart Array P400


[root@test /]# dd if=/dev/vg_99/lv_share of=/dev/null bs=1024 count=10240000
10240000+0 records in
10240000+0 records out
10485760000 bytes (10 GB) copied, 40.097 s, 262 MB/s
[root@test /]# dd of=/dev/vg_99/lv_share if=/dev/zero bs=1024 count=10240000
10240000+0 records in
10240000+0 records out
10485760000 bytes (10 GB) copied, 500.826 s, 20.9 MB/s

So, 262 MB/s reading vs 20.9 MB/s writing.