configuring RAID10 for data in Amazon EC2 cloud?

does anyone have any tips on this? Linux Software Raid doesn't seem to be doing a very good job here, but i may well have missed something.

i did a fairly naive setup using linux software raid on an amazon linux instance,
10 volumes (8G each), (WAL on a separate EBS volume) with the following setup:

mdadm -v --create /dev/md1 --level=raid10 --raid-devices=10 /dev/xvdg /dev/xvdh /dev/xvdi /dev/xvdj /dev/xvdk /dev/xvdl /dev/xvdm /dev/xvdn /dev/xvdo /dev/xvdp

pvcreate /dev/md1

vgcreate vg-pgdata /dev/md1

vgdisplay vg-pgdata

lvcreate -L39.98g -nlv-pgdata vg-pgdata

this particular instance is running about a factor of two slower than a simple single disk instance. both the single disk instance and the one with RAID10 for ~postgres/data/base
started from amazon m1.xlarge instances.

postgresql version is 8.4.9, using a simple pgbench test for 600 seconds; the single disk instance shows this:

dbDev, single disk, shared_buffers=4GB, effective_cache_size=8GB
disk mounted noatime, readahead 4096, other stuff default

-bash-4.1$ pgbench -T 600 bench
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
query mode: simple
number of clients: 1
duration: 600 s
number of transactions actually processed: 535018
tps = 891.696072 (including connections establishing)
tps = 891.704512 (excluding connections establishing)

and the RAID10 instance shows this:

dbQA, wal+raid10 setup, ext3 for WAL, ext4 for raid10,
shared_buffers=2GB, effective_cache_size=3GB
readahead 10240, wal&raid mount noatime, journal=ordered
vm.swappiness=0,vm.overcommit_memory=2, dirty_ratio=2,
dirty_background_ratio=1

starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
query mode: simple
number of clients: 1
duration: 600 s
number of transactions actually processed: 261513
tps = 435.854738 (including connections establishing)
tps = 435.858853 (excluding connections establishing)