Comparative tps question

Hi, I've just been benchmarking a new box I've got and running pgbench
yields what I thought was a slow tps count. It is dificult to find
comparisons online of other benchmark results, I'd like to see if I have
the box set up reasonably well.

I know oracle, et al prohibit benchmark results, but was surprised that
there doesn't seem to be any postgresql ones out there..

Anyway, the machine is a Dell R720 with the data on a raid 10 using 8x
intel 320 SSDs and a mirrored pair of 15k SAS HDDs configured for the
pg_xlog, both on a dell H710 raid controller, in addition it has 64Gb of
1600Mhz memory and 2x E5-2650 processors (with HT=32 cores). The arrays
are all setup with XFS on and tweaked as I could. The drives are 160Gb
and overprovisioned by another 15%.

I'm running postgresql 9.1 on ubuntu 12.04

bonnie++ (using defaults) shows about 600MB/s sequential read/write IO
on the main data array, this doesn't seem too bad although the specs
show over 200MB/s should be achievable per drive.

pgbench (using a scaling factor of 100 with 100 clients and 25 threads)
gives an average of about 7200tps.

Does this look acceptable? Instinctively it feels on the low side,
although I noted that a couple of blogs show
(http://www.fuzzy.cz/en/articles/ssd-benchmark-results-read-write-pgbench/
and
http://it-blog.5amsolutions.com/2010/08/performance-of-postgresql-ssd-vs.html)
show around 1500tps for a single ssd, so maybe this is what is expected.

The interesting param differences from the postgresql conf are:
share_buffers=6Gb
work_mem=64Mb
max_stack_depth=4Mb
random_page_cost=1.1
cpu_tuple_cost=0.1
cpu_index_tuple_cost=0.05
cpu_operator_cost=0.025
effective_cache_size=40Gb

I'd be happy to provide any other configs, etc assuming the tps values
are way off the expected.

Thanks

John

ps. the number of "safe" ssds available in the uk seems to be rather
limited, hence the intel 320s which I probably aren't as fast as modern
drives.