What is your phisical limit?
Good code can't improve your bandwith, as i believe the problem is in bandwidth there are mainly 2 ways:
1) Using compressend data for trasmission;
2) Using a p2p structure: Let S be the main server and A,B,C,D,E the destination:
1) S send to A
2) A send to C
2) S send to B
3) C send to D
3) B send to E
Consider also rsync....