edit the source code and change the # of outgoing connections.  i do 500 on a dedicated gigabit line.  this bursts up to 60-90MB/s for a second or two when a new block hits the network.

i tested it on my home connection once & after a new block, it'd saturate the upstream for a good 30s or so.  i guess that'd be a good way to slow down propagation, having 500+ connections and then limit upstream to 512Kbps or so.