Running S3 Uploads in Parallel

I've been trying to shave off a few seconds when uploading large batches to AWS S3. Parallel seems to do a wonderful job in this situation!

164 pdf files of varying sizes on a dual-core i7
25 seconds: s3cmd put *.pdf s3://bucket-name/goes/here/
25 seconds: parallel --jobs 4 s3cmd put {} s3://bucket-name/goes/here/ ::: *.pdf
18 seconds: parallel --jobs 8 s3cmd put {} s3://bucket-name/goes/here/ ::: *.pdf
17 seconds: parallel --jobs 16 s3cmd put {} s3://bucket-name/goes/here/ ::: *.pdf
15 seconds: parallel --jobs 20 s3cmd put {} s3://bucket-name/goes/here/ ::: *.pdf
16 seconds: parallel --jobs 24 s3cmd put {} s3://bucket-name/goes/here/ ::: *.pdf
16 seconds: parallel --jobs 28 s3cmd put {} s3://bucket-name/goes/here/ ::: *.pdf