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.
That's probably a pretty bad idea, considering you're then eating up connection slots when you don't really need to, since you're perfectly able to be reached by other users. Editing your own outgoing connections beyond the standard 8, means there's less room for those who cannot forward port 8333 when behind a NAT.