While there are currently no independent tests performed on MonoTorrent, i have conducted my own tests while writing the client. I'm happy to say that the initial results are very good. For anyone who wants to run these tests themselves, here is how i took my readings.

Memory Usage
Each client tested was started, minimised, then maximised and then a Ubuntu torrent started immediately. The reason for the minimising and maximising is that .NET applications reserve a chunk of memory immediately on starting up and won't release the memory until a predefined number of garbage collections has been reached or the application is minimised. By minimising and maximising, all unneeded memory is returned to the OS. Unmanaged applications do not have this 'problem'.

The torrent was then allowed download for 5-10 minutes until memory usage seemed to stabalise. These numbers where then recorded as final memory usage.

CPU Usage
This one is harder to measure objectively. To test this, i got a small torrent (~100MB) and let each client download the entire torrent. I recorded the amount of CPU time each client used while downloading the torrent. This is the value recorded below.

Screenshots of the results can be found here.
Client Memory CPU
Azureus 40MB ?
MonoTorrent 8.5MB ?
uTorrent 4.6MB ?