2 x MSI R9 280x GPU Hash Rate - Is this normal?

Hi all,

Just a quick question regarding the hashing speeds I should be expecting from my mining setup.

I am currently running the following configuration, though am only getting a 28.6MH/s hash rate. From the research I have done on the forum I was expecting something in the 40MH/s speed, give or take a little.

1 x AMD Phenom Quad Core CPU
16GB DDR3 Memory
1 x 120GB SATA Drive
2 x MSI R9 280X GPU - running at stock speeds
700W PSU (540W being drawn at the wall)
Windows 7 x64 OS

I am running Eth Proxy pointing at the DwarfPool Stratum instance. I had tried EthPool but was receiving an even lower hash rate.

The Windows Mining version I am running is: 0.9.41 - this was the latest build I could find.

I have also made sure that both GPUs are mining, I can see this via the mining and proxy UI, along with the MSI Afterburner util.

I have been mining for 24 hours constantly, to ensure that the hash rate given has time to average out.

Is this 28MH/s hash rate I'm getting typical for a pair of MSI R9 280x GPU's, as from what I've read I should be seeing something closer to 40MH/s?

Any feedback much appreciated.

Thanks, Simon