In S3 tests on c3.8xlarge instances, I've seen 8 Gbps throughput on both uploads and downloads using parallelized requests. Testing with iperf between two of the same instances maxed out about 8 Gbps as well so the throughput limitation is likely EC2 networking rather than S3.

These tests were done over a year ago so bandwidth limitations on EC2 may have changed since.

This testing was with https://github.com/rlmcpherson/s3gof3r