For internet speeds it's always the "only as fast as slowest link" rule. The speed of uploads and downloads depends on your own internet connection, because almost all the time I have more bandwidth than the residential internet that my clients have. So long as all the links between me and the client are OK, and the client has average internet or even better-than-average, the client will hold the slowest link.
DontCare4Free is correct, because the game sends packets at 32 per second (default, set by pref) and at a max of 1024 bytes (also by pref), it means that no matter how good your internet is you can only upload to your Blockland server at a certain speed, which is 32KB/s or 256kbps. I recommend doing the FTP and load bricks method that I mentioned in the OP.