I've noticed pretty slow tethering speeds as well. I'm currently on stock ROM and stock radio... haven't run an actual test, but will when I get home and will post an update. Surely there's gotta be a reason because on the device itself my speeds seem pretty fast (anyone know of a good speed test to run on the actual phone?)
Edit: Found one -
http://dslreports.com/mspeed
w/ a 1MB test file (you have to choose a smaller file, get the error that its too fast and then click the link to choose a larger file) I ran it 3 times, got measured at 544, 1087, and then 750kbps (which averages out to 793kbs).
I'm in Austin, TX and at the time of the test (while at work) I had 3-4 bars inside my building. So, looks like its the device itself, not just a result of the tethering... guess it seems so much faster because I'm usually loading mobile sites on the phone vs full sites while tethered. :P