You can get increase by modifiying the file sys:prefs/env-archive/sys/net/netstatck.config. Edit this file and at the bottom add those two lines, if not present, tcp_recvspace and tcp_sendspace. That would be so :
On my 2 computers, pegasos 2 and powermac mdd I have increased the values as indicated and on a 16 mbits connection by my provider, orange, and I reach it or overcome it to 17 mbits stable. You can try to increase these values more but when downloading big files, as chrysalis.iso for example, the rate must stay stable, near of the highest. If you obtain a high value only a few seconds and then that falls a lot then your values are too high.