If you are running windows 7, One possibility is that you have Power Options set to "power saving mode" which might  be setting the 6950 into its lowest power state.

I'm using poclbm and getting a hash rate of 310 Mhash on the 6990 with the bios in default setting, on the unlocked 6970 I get around 320 Mhash.
I've used this guide by roulo http://bitcointalk.org/index.php?topic=3359.msg47174#msg47174
And then continuing with this guide by humble http://bitcointalk.org/index.php?topic=3356.msg47489#msg47489

I used 11.3 Catalyst (latest available) with the latest APP for linux available.
Disabled autoupdates through update manager (after finishing installation) (caused problems after an autoupdate)

in order to overclock/underclock the memory I used AMDOverdriveCtrl
http://sourceforge.net/projects/amdovdrvctrl/

launched 1 for each gpu using (AMDOverdrive ctril -i <adapter num given by the --help flag>) in the bash.

My only gripe is that on a seperate (everyday usage) rig my other 6970 gets 345 Mhash @ 900Mhz on windows where I can push it further (than 840Mhz  on linux) with MSI Afterburner, and lower mem clock lower than 725Mhz (linux).

So if I can sort out the little problems in windows or get hold of a nice alternative mining software, I'm pretty sure my hashing rate would go up.

6950 reference is pretty easy to unlock with atiwinflash + the RBE tool (takes about 5mins), I did it under windows, just get the bios image using atiwinflash, load it up with the RBE, go to the additional features tab where you then select unlock 6970 shaders, load it back on on the 6950 with atiwinflash(unlock bios if you have to) restart , done.

I assume you mean 6950, as I dont have experience with 5850.

I'm running a 6990 + 2 6950(unlocked) in Ubuntu 10.04 just fine with 11.3....

I believe 6990s are the way to go, as the VLIW4 architecture is the future for AMD and they have some potential in terms of developing their drivers.

One of the main reason the 6990 has approx 50w more power draw than the 5970 is because of the 4 GB memory, if you underclock that, you should have lower power consumption than the 5970.

There are many reviews that use the unleashed mode (hardly any benefit) and thats where you get the apparent massive power draw from.

On top of that the 6990 also features power tune, I'm using it on my 6950s (-11%) it reduces power consumption substantially and doesn't affect the hash rate.

Oh and 6990 has resale value and is a monster when gaming !