My server uses a 100Mbps shared port with 10Mbps nominal guaranteed BW. The highest speed of upload to Amazon S3 that I've seen from the server is 2.5MB/s. What does this indicate with respect to the server's actual bandwidth capacity?
It means absolutely nothing. Your provider can only guarantee a certain bandwidth to the edge of their network -- it's entirely possible that network congestion is going to cause problems beyond their bounday. Also, an upload to Amazon is likely to be using a single TCP connection, and the bandwidth-delay product could very well be biting you in the butt.