I didn't do any file transfer benchmarks, but when I had my new 802.11n Extreme setup to allow b/g connections, my MBP's connection speed to the Extreme dropped from 300mbps to 160mbps.
160mbps is still faster than 802.11g (52mbps), but it's still almost 2x slower than 802.11n can connect when there aren't any b/g devices around.
What I ended up doing was setting my new Extreme to a 5ghz 802.11n-only network and using my old Extreme to run a separate wireless network for the 802.11b/g devices I still had. The b/g network has a limited range, so I have an Express on the end of the house connected to old Extreme via WDS. The range of my new Extreme still exceeds the extended b/g network at my place. I don't know if it's the 5ghz or what...
I set the old Extreme to bridge (so there was an ethernet cable that ran from a LAN jack on the new Extreme to a LAN jack on the old Extreme), so even though there were two totally different wireless networks in my house, they were the same TCP/IP network. So regardless of which network you connect to, you got an IP address from the same DHCP server (the new Extreme), which means the devices had zero problems talking with each other, even if they were on different wireless networks.
I know that's not how you want to setup your network, but maybe there was something in there that you could use?
I've had multiple printers and USB drives connected to the new Extreme (via a powered hub) and they all worked fine. My only complaint is that the speed of copying file to/from the USB drives was slower than I had expected. It's quicker for me to wirelessly copy files to a USB drive that's hanging off of a Mac mini (that's connected to the new Extreme via a ethernet cable). I mean, the speed of the Extreme + USB drives is definitely usable, .. I was just hoping for a little more mbps.
