Actually the download speed for the user, to clarify, would be whatever the upload speed is available on your seedbox's Internet connection (first)... then whatever the download speed is on your user's end.
Meaning, if you have a top upload speed of 5MBps, your seedbox is using 4MBps to seed torrents to the Internet, then you have only 1MBps "free" for your "user" to download a file from you. So if your download user had a 50MBps download Internet connection, the best they could do with downloading a file from your seedbox is 1MBps (and probably less due to data overhead/etc).
If this is your home Internet connection shared with your seedbox (like I have), then you could run into trouble with your Internet surfing being a little spotty with your upload side of your Internet connection maxed out. Unless you have priorities/bandwidth limitation rules setup on your router.