LimeWire refers to speeds in Bytes not bits. Are you sure the speeds are bits as LW showed downloading. 50 KB/s is equivalent to 400 kbit/sec, and 500-1000 kbit/sec to 4 -8 MB/s. And are you sure you downloaded the 500-1000 from a single host or multiple hosts? Usually when I'm getting those speeds I am downloading from 8-10 hosts.

Have you done a speed check for your 1000Mbit/sec line to see what the upload capacity is?

Sometimes the problem is ISP hops between you & the other location. If it's going through ISPs who are having heavy traffic this can affect things. Also some ISP's may shape p2p sharing bandwidth in part to cope with heavy traffic.