With an original 500 GB Time Capsule, I've copied a relatively large (100 MB) file to my TC and from my TC to my Desktop using the Finder and measured the throughput using IPNetMonitorX's ethernet interface monitor feature. Over gigabit ethernet, I'm seeing about 28 MB/sec down and about 18 MB/second up, which seems quite respectable. When I enable a 5 GHz 802.11n-only link (Network Utility reports a 300 Mb/sec link), however, I see a reverse asymmetry: about 8 MB/sec up and only 2-3 MB/second down.
While I expect the throughput over 802.11n to be vastly less than over gigabit ethernet, 1) why would uploads to the TC be faster, and 2), why is the download throughput so poor? I'm not seeing send or receive errors, or collisions, by the way.