I can somewhat second the OP, the problem is abit different than described above though. If I configure N Only the overview shows 300 Mbps rate for me with 0 clients connected.
Looking into it a bit further I noticed that the GUI Basic - Network seem to have changed in the latest release v1.28.9052 MIPSR2-beta23 K26 Std
(build 52 - 10/15/2010)
I actually just bought two wrt320n 5 days ago and flashed them straight away with the latest TomatoUSB available at the time, beta22, Build 51 - 10/06/2010 selecting the Kernel 2.6 (experimental) for MIPSR2 - NoUSB Std. I have been running Tomato for a long time on wrt54gl's and it has been working like a dream.
After the initial flash finished I logoned to the Tomato gui and performed a restore default with "Erase all data in NVRAM" and after that finished up the setup.
I'm the type that usually documents everything I do so I made a document with the settings I edited in the webgui to have my WDS up and running.
The WDS was working fine until I upgraded to beta 23 when the speed over the wireless network dropped to a third of before.
In my documentation I had this option:
Radio Band: 5GHz
When I look in the gui in the latest release (v1.28.9052 MIPSR2-beta23 K26 Std) this option is not available at all, and the Wireless channels available are ONLY in the 2.4GHz range.
Just to make sure I hadn't written that out of my mind, I flashed one of the routers back to v1.28.9051 MIPSR2-beta22 K26 Std and the Radio Band option among with channels starting on 5.x are once again available.
So I think that is the problem.
Is there any more specific output from the unit that I can give to help troubleshoot this?