Jits, here is a possibility maybe...
Have a dedicated server stricltly for downloading maps and make the download speed not capped.. When you dont have a map the game switches from your normal server to the dl server, dls the map at normal speeds, then reconnects you to the normal server. From what I understand, the only reason there so slow is so that it doesnt lag the players that are playing on the server, but if there is no one playing on the d/l server, then theres no lagging them.
Would that work?