1) Time Machine can be used over a network, apparently. You need to tell it to show you the various options, though. According to this website you do this at the command line:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
and voila.
2) Gigabit network efficiency can be increased drastically, according to this. The article references a site at PSU that hosts an NPAD test server