I think I've figured out what was wrong with the network.

For some reason, a long time ago, I turned on IPV6 for each of the network interfaces on my Macbook Pro - both wired and wireless.

Turning off IPv6 for each interface fixed the data rate issue I was seeing.

I guess that because my old Linksys didn't support IPV6, the issue didn't present itself until another IPv6 capable device started chattering on the network (the Airport Extreme supports IPv6). Turning off all IPv6 support immediately fixed the issue.

I've since re-upgraded the firmware on each of the Airport Express' and the Airport Extreme, turned back on Wide Channel support, upped the multicast rate, etc. And each step of the way, the network performance was awesome.

I'm sitting here now listening to Led Zeppelin over Airtunes and I'm still getting approximately 5mbps down and 500kbps up.