Rsync would be better than SFTP, because you can start copying the file before it is done. I have large files copied to my laptop from my seedbox almost as soon as the download completes on the seedbox.
I haven't used a seedbox, but it seems like the best arrangement would be to download the files from your seedbox using BitTorrent over a VPN. Your local client would only accept local peers (which the seedbox would be, virtually), so you aren't compromising your security, but you get your files almost as fast as your seedbox.
I'm pretty sure clients can be configured to make this work, but I haven't looked into the details.
Rsync sends only the changes, wherever they are in the file. So if you set up a "while 1 rsync remotefile localfile" kind of script it will converge on the correct version of the file with minimal overhead.