FTP app that can monitor a local folder
Transmit can achieve what you're looking for thanks to its Transmit Disk feature.

Once you have specified a remote location as a favorite (analogous to bookmarking it), you can use Transmit Disk to create a local volume with the same name as the favorite. Any changes made to this volume's files are mirrored to the server. The two are kept in constant sync.

According to Panic's website (and my own usage), the files are uploaded even if Transmit.app itself isn't running. Transmit Disk runs as a separate background process. On my system, with one volume mounted and shortly after requiring a sync, this process is using 0% CPU and 13MB of memory. Performance, it seems, is not an issue at all.