While being a supernode does use some bandwidth, I am guessing that most of your bandwidth was going towards the uploads instead of just the network connections. If that was the case then setting up your client as a leaf might not make a difference as it would just allow for more bandwidth to be spent on uploads.

As for the connection attempts, I would agree with Taliban that it is most likely bearshare clients.
I noticed the same thing happening with all the connection attempts. I made a program that just listens for connections and logs them. After closing the gnutella client and starting this program I was pounded with download requests. Almost all of them came from bearshare clients and were constant (ie client connected, i closed connection, it would reconnect ).