You know about asymmetric connections? Your download is a LOT faster than your upload. No way to increase your upload speed than ask your ISP to upgrade... or get a symmetric connection where up- and download speed are the same.
Example: My line is DSL 6000, means
download speed is 6016Kibit, equals 752KiB > that's what I get from the 'net
upload speed is 512Kibit, equals 64KiB > that's what other people get from me.
It doesn't matter, if FTP, HFS, P2P - practically your server is SENDING the file to the client, all they ever will get is your UPLOAD speed, which is not very much in most cases.
Ask your ISP about your upload speed if you don't know. If it's close to what your clients get from HFS, forget it. You can't improve it further.
MarkV
edit: In case you wonder what KiB really means, read
http://en.wikipedia.org/wiki/Kibibyte